EDIT: I just noticed that you were on Cingular. Nevermind BUT you can still use Blackberry Messenger.
Well, if you are using a BB 7290 on the ATT network, you will not be able to use a third party chat application. I spent many hours trying to get it set up on mine without any luck. If you are on ATT, you must be on a BES in order for this to work. There is another option called Blackberry Messenger. That is currently what I use. You can get this at http://www.blackberry.com/messenger
I hope this helps.