Q: Messages on mac

Hi,

So when you select a Contact to send an iMessage to do they appear red with an error alert saying "Not registered with iMessages" ?

Does this happen for every Contact ?

Top left is the Apple Icon and it is a Menu if you click and hold it.

Select About this Mac.

In Yosemite and El Capitan the Overview tab show you your Mac's serial Number.

Is this visible ?

(In Mavericks (which you list you are using)  you have to click the line that tell you the OS version twice to be able to see the Serial Number).

IF it is not there you will have to contact an Apple Store (most likely visit) to get it put back.

If it is there go to System Preferences > Users and Groups

Either Enable the Guest account or create a New Mac User for test purposes.

Login to the Test account you are going to use.

Set Up Messages fro the iMessage account.

Does it work in this Mac User/Guest  account ?

Report back at this point as the answers from here depend on what you find  and trying to decribe all the options would make the reply unwieldy.

8:51 pm      Thursday; August 25, 2016