Certain menus in iChat change what is active or in some cases change the menu depending which of the iChat windows is the Front one.
The Add Buddy will grey out in the Buddies menu if the front window is a text Chat window and the Buddy is already in the Buddy List.
It should change to read the Buddy's name as well.
If they are not in the Buddy List then their Name should still appear but the line should now be dark text (Active)
The Plus sign to Add Buddies or groups at the bottom of the Buddy list should never grey out. (Apart from the Bonjour List that self discovers local Buddies and does not have the Plus sign)
If you are not logged into the AIM server the Buddy List will not drop down and fill in with at least On-Line buddies.
However if you are someone that hides the Buddy List for screen space and bring iChat to the Front anyway the Add Buddy item will appear greyed out.
As the Login is largely controlled by the com.apple.ichat.AIM.plist this is why this one should be deleted to correct any faulty login and iChat should be restarted to do so.
If this does not cure it and neither does deleting all the .plists and the Buddy List appears to be displaying properly then we have to look elsewhere.
Are you Logged in to AIM and do you see the rest of your Buddies ?
Do any of the scenarios seem to fit what is going on for you ?
6:39 PM Saturday; May 17, 2008