Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
For 3GS, to use custom firmware (from pwnagetool or snowbreeze), the phone needs to be already pwned JB. Last pwned JB was Redsnow 3.1.2 or Spirit 3.1.3 + Spirit2pwn (Cydia). Thus, question, is your phone currently pwned JB?
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.