using your BB browser go to the mobile.blackberry.com website and if you scroll down to IM services it should be somewhere there, i dont remember 100% where..,. also would like to mention i dont think its designed by RIM it might be a third party app
SMS to me would seem to be much less of an "instant" messenger system. i know when i use my JiveTalk and send a message, it is sent and recieved much faster than an SMS