Call Customer Service and make sure that they activate MediaNet Pay Per Use on your line.
The preinstalled IM client requires data to login and update buddy lists. Data associated with using the preinstalled IM client is NOT charged.
So if you have medianet blocked on your line, you need to unblock it in order for the preinstalled IM clients to work. Messaging thru the preinstalled AIM client count as TEXT messages, so make sure you have a texting plan.