Unlock iPhone

First you have to jailbreak your iPhone 3GS. Read my RedSn0w tutorial on how to do that first. It should take about 10 minutes and it is a necessary step to be able to unlock your iPhone.

After jailbreaking your iPhone 3GS, follow these simple steps to install UltraSn0w:

Launch Cydia.
Add the following source to Cydia repo666.ultrasn0w.com (note there is a “0″ in sn0w, not an “o”).
After installing this source in Cydia, search for “UltraSn0w”.
Install UltraSn0w and reboot your iPhone.

That’s it