Using RIM's Yahoo! Messenger Client on Cingular Wireless BlackBerry Handsets
Try this guide from this site. It will let you install RIM's client on your Cingular/ATT Pearl. It looks complicated at first, but it takes about 5min tops and is very simple even if you don't have a lot of tech knowledge. Basically:
1. Download the file with the link given
2. Use Desktop Manger to "Restore" (aka load) the file to your Pearl
3. Re-register your handheld on the network (click on an option in your option menu)
4. Log into your BIS/BES account from the link given and click a button (should already know your login.password)
That's pretty much it assuming it goes smoothly (mine did). You may need to do a hard reset or something but not likely.