Could be a number of things.
If you see the Balloons (presuming you're using Balloons as the style) but cannot see the text I would check iChat Menu > Preferences > Messages and make sure the Font is not the same colour as the Balloons.
If you are using Balloons as the Style (View Menu > Messages) then change it to one of the other style choice you have.
(Actually iChat 3 does not have the Messages sub menu only "Show as Text" or "Show as Balloons")
The later versions also have different Style options.
Does changing the Style change whether you see the IM properly ?
I presume what you actually mean is that the Chat window does not actually show the Balloon or other Style (IM) in the chat and that you do not see the Incoming IM either.
Try the Style change for this as well.
The Styles are run from Plugins and if one is "faulty" then another may work.
If it turns out the Style choice is the issue you will have to reinstall the app. (there is no real way to (re)install the plugins for the style choice.)
The Instructions include the methods for iChat 3 through 5.
After you have reinstalled and before your start up the App you will need to download and install the Combo Version of the last Update to the OS you did so that iChat gets updated.
iChat receives updates in some of the OS updates.
Once the OS has been updated iChat need to keep pace with this.
Reinstalling from the Install DVD is often from the first version of the OS.
The combo updates include all the updates from that DVD install disk to the "Present" you happen to be using.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
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