The answers are all within this thread:
Go to
http://www.t-mobile.com/bbupgrade
Download the latest 7100t code (v128). Execute the downloaded program.
Locate "Vendor.XML" on your C: drive and delete it.
Start Desktop Manager; it should ask to upgrade the BB.
Try IM again. There are many, many threads about problems with IM On this board. Many people use third party programs.