There's no really elegant way of doing IM built right into the Treo 600. However, there are a few add-on software items that you can HotSync into a Treo that will do IM.

Although no endorsement is being given here, perhaps the most popular IM program for the Treo 600 (both GSM and CDMA versions) is "VeriChat", available for a yearly fee of $24.95 here:

http://www.pdaapps.com/verichat/default.html

... this program does all 4 of the big four IM brands. It even does some of the more tricky things like allow Mac OS X "@mac.com" users log into AIM.