Hi,
If your AIM valid Screen Name happens to be an Apple ID then it is likely that only if it is one Ending in @mac.com will it work in Leopard and iChat 4.
This is because when Apple went to iCloud they also changed the way iChat 6 and later Mesages logs in.
Both these version also contact Apple and this then allows the AIM Servers to "read" the password.
This Dual Login does not happing in earlier versions which means the Apple Servers are not contacted and then the AIM servers are blocked from checking the password.
It also means you cannot use an @me.com or @icloud.com ending name in the AIM for Mac App, the AIM Settings login page or login to AIM express on any Browser on any device.
For the Text not showing try this test change first.
Open iChat.
Go to the View Menu
Select the Style option and change it from Balloons to anything else.
Actually Quit iChat and restart the app.
Does iChat see the text when not using Balloons ?
If so you need to reinstall the whole App. The Balloons Style is actually run from a Plugin but you can only get it by reinstalling the whole app.
See the Users Tips tab at the top of the Discussions list.
EDIT Direct link
Basically tick in your Original Install DIsk.
Select the Option Installs foldder and launch the OptionalInstals.mpkg
Pick out iChat only and Install
Download and Install (over what you have) the COMBO Installer for OS X 10.5.8 (THIS is needed to update iChat to the corerect version of OS X 10.5.8)
9:58 PM Monday; August 19, 2013
 iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
 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
"Limit the Logs to the Bits above Binary Images."  No, Seriously
Message was edited by: Ralph Johns (UK)