Q: Why won't iMessage load on my MacBook Air?

It shouldn't take more than a few minutes. Press the return key, if you haven't already done it. If you have, post a screenshot of the Terminal window.

Start a reply to this message. Click the camera icon in the toolbar of the editing window and select the image file to upload it. You can also include text in the reply.

I'll wait on your continuation with Linc.

Linc's method using the Terminal is quicker  than my clunky navigation to individual files to check them out.

7:30 pm      Saturday; December 28, 2013

  iMac 2.5Ghz 5i 2011 (Mavericks 10.9)
 G4/1GhzDual MDD (Leopard 10.5.8)
 MacBookPro 2Gb (Snow Leopard 10.6.8)
 Mac OS X (10.6.8),
 Couple of iPhones and an iPad

The command finished. The output is on the Clipboard, or it was. If you've copied something else, run the command again.

We are hampered by the facts that there are apparently no log messages and that you couldn't boot in safe mode for some unknown reason. However, since Messages seems to work OK in the guest account, I suggest the steps below.

Back up all data.

Quit Messages if it’s running. Relaunch it and test after each of the following steps. If the problem isn't resolved, quit again and go on to the next step.

Step 1

Make sure you know the ID and password you use with iMessage. Launch the Keychain Access application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad. Click Utilities, then Keychain Access in the icon grid.

Use the search box in the toolbar of the Keychain Access window to find and delete all items with "iMessage" or "com.apple.idms" in the name. Log out and log back in.

Step 2

Hold down the option key and select

Go ▹ Library

from the Finder menu bar. Move the following items from the Library folder to the Trash (some may not exist):

• Caches/com.apple.Messages
• Caches/com.apple.imfoundation.IMRemoteURLConnectionAgent
• Containers/com.apple.iChat
• Containers/com.apple.soagent

Leave the Library folder open. Log out and log back in.

Step 3

Go back to the Finder and move the following item from the open Library folder to the Desktop:

• Messages

Note: you are not moving the Messages application. You’re moving a folder named “Messages.”

If Messages now works, delete the Messages folder on the Desktop. Otherwise, quit Messages again. Put back the folder you moved, overwriting the newer one that may have been created in its place.

Step 4

In the Preferences subfolder, there may be several files having names that begin with any of the following strings:

• com.apple.iChat
• com.apple.ids
• com.apple.imdsmsrecordstore
• com.apple.imessage
• com.apple.imservice

Move them all to the Desktop. There may also be a file with the name "com.apple.imagent.plist". Move that to the Trash.

Also in the Preferences folder, there's a subfolder named "ByHost". Open it and do the same thing.

Log out and log back in. Test again. This time Messages should perform normally, but your settings will be lost. You may be able to put back some of the files you moved to the Desktop in this step. Relaunch and test after each one. Eventually you should find one or more that causes Messages to malfunction. Delete those files and recreate whatever settings they contained.

If the issue is still not resolved, quit Messages again and put all the items you moved to the Desktop back where they were. You don’t need to replace the items you moved to the Trash. Stop here and post your results.

If you later decide that you don’t like the results of Steps 3 and 4, you can undo them completely by quitting Messages and restoring the items you deleted in those steps from your backup.

HI,

Have the deleted .plists been recreated ?

This might hint that a containing folder such as the Preferences  or the ~/Library itself is not allowing you (as the computer) to write to the .plists

I tend to do it this way.

In any Finder window I then use the View menu the Show Path bar.

I then use the Finder's Go Menu whilst holding the ALT key down and selecting the Library that appears.

Then I navigate to the Preferences folder.

So that com.apple.ichat.plist, when highlighted looks like this.

The com.apple.ichat.AIM.plist and the other service names including the Subnet one (for Bonjour) are where the accounts for those services are held.

The LSSharedFileList  only holds the Recent List items.

The Status Messages one holds the Saved Status Messages you may have created.

The route that the app uses to write to these plist is different though from Mountain Lion and Messages 7.x.x

Click on the Library item to open that folder.

Open Containers/Com.apple.iChat/Data/Library/Preferences

You will notice that these are mostly aliases pointing to the ~/Library/Preferences you were in  a minute ago.

Highlight the  com.apple.ichat.plist (in the main Preferences folder)

Now Use the CDM + i keys together (Get Info)

It will say at the Bottom if your Own it and if you have Read and Write permissions.

Right click all the enclosing folder and see that you own those as well

9:38 pm      Saturday; December 28, 2013

  iMac 2.5Ghz 5i 2011 (Mavericks 10.9)
 G4/1GhzDual MDD (Leopard 10.5.8)
 MacBookPro 2Gb (Snow Leopard 10.6.8)
 Mac OS X (10.6.8),
 Couple of iPhones and an iPad

What happened when you tried to boot in safe mode?

There's a link to a support article in the comment where I suggested it.

Are you quite sure that no messages appear in the Console window when you try to launch Messages?