Hi and welcome to Everything Treo.
In terms of IM, I'd recommend
VeriChat. It will allow you to use
AOL IM, Yahoo IM, MSN IM and IRC.
It's a great app even if you just want to use it for AOL IM.
When you download it, you should find install directions.
All Palm apps use a .prc file. Not sure if there are supporting
files for VeriChat. The .prc needs to be sent to your Treo during
the HotSync process.
Enjoy!
Chris