Check out bbtext2im. Its still beta, but its on 3rd release. Not perfect yet, but worth it, and the creator is always improving the app. Its just a matter of time before just about perfect
Posted from my CrackBerry at wapforums.crackberry.com
OK, first off BBText2IM is a threaded SMS view app. There's no real IM going on here. It's just txt messaging in a different view. What I'm assuming you want is AIM/Yahoo!/GoogleTalk something like that. I don't think I've heard of a BB not having those apps preinstalled. Who's your carrier? Do you have DM and your OS installed to your pc? Pretty sure those apps would be included with your OS. Open DM, goto application loader, then add/remove (DM 4.5), look for AIM, or whatever IM client you wanted on this list, make sure the box is checked.