I deleted all the AT&T crapware, so I'm not sure what IM programs were included with the Tilt.
But did you try the official AIM for Windows Mobile program?
AIM for Windows Mobile
Personally, if I decide that I want to use IM on my Tilt, I'll probably buy the IM+ All-in-One Mobile Messenger ($40) from shapeservices.com . I'd prefer to have all my IM services (AIM/ICQ/MSN/Yahoo/GoogleTalk) unified into one program.