Don't have permissions on my own Mac?

I don't understand what you are trying to do. Which Preferences folder are you trying to change or have Permission to change with full read and write? Why the screenshot of Mail? To what exactly did you add yourself as user?

First, I think you went to the Preferences folder in the wrong Library. The ~ in the path indicates your home folder Library. The screenshot above with the mucked up Permissions is the second level HD Library. This is what you should be seeing instead.

And I think there is a basic misunderstanding about the admin group. You may have administrator privileges for your account, but that's not the same thing as the admin group, for purposes of Unix Permissions.

For the Permissions you messed with, get BatChmod, which has an easy to use interface for changing the actual Unix Permissions.

http://www.lagentesoft.com/batchmod/index.html

This is what /Library should look like. Only hit Apply. Do not hit Apply to enclosed or Clear ACL's or anything else. Go to the second level HD Library and drag that folder into the field next to File.

Not being able to save a preference setting may be the result of an overfull startup disk. Do a Get Info, right-click, on the Macintosh HD folder, or whatever you may have named it. to show capacity and available.

The ~/Library/Preferences is a User account folder holding your preference file

com.apple.mail.plist

is the one for Mail, it's not a System file, but a user account prefernence file.

Do the #6 Reset Users Permissions, then do the #12/#13 OnyX cache cleaning and Verify preference files and list the corrupted ones and delete them and reboot, they will rebuild.

..Step by Step to fix your Mac

Deleting the System Preference or other .plist file

If there are a lot, it might be just best to use the Disk Utiltiy > Secure Erase > Zero from the 10.6 disks and reinstall OS X 10.6 over again from scratch.

I suspect your reinstalling over a old drive that has some bad areas on it, the zero erase should map those bad areas off and ensure a better install.

And I think there is a basic misunderstanding about the admin group. You may have administrator privileges for your account, but that's not the same thing as the admin group, for purposes of Unix Permissions.

It's just that this can get a bit complicated. The account administrator is in the Unix admin group, but that doesn't necessarily mean that the admin group for a particular item will give the administrator full r w x privileges. And you don't need write privileges to run or use a program. "Folders and files inherit the group of the folder in which they were created, regardless of the user's default group."

And the Ownerhsip/Permissions displayed in Get Info gives very little information. It is only a pale reflection of the actual Unix permissions.

For more, see

https://support.apple.com/kb/HT2963

Note: I wouldn't go full bore with any of what ds store is suggesting until you try my suggestions above and find they don't work. And you don't need to do a reset for users permissions unless you see ~/LIbrary/Preferences isn't what the screeshot above is showing.

And clearing out some space on the drive if it is unusualy low, may be a fix for the initial problem.