I had this same problem, I called AT&T this is what they told me and help me get an IM that works with AIM, MSN, and Yahoo.
go down to your media browser (media Network icon on the bb 8310)
go down to mail and messaging go down to mobile IM, click on it
its called OZ messaging it works awesome... didnt mess with my blackberry at all..
media browser/mail and messaging/mobile IM then download login
sometime i find that i have to refresh my contact list after login but no big deal... and its FREE