Log in to google voice at google.com/voice, click the little tool icon on the right side, and if you hit the edit button, it will give you a string of characters very similar to this:
##004#612-555-1234## with the number being your number of course.
You punch that in the dialer and it will change your settings automatically. Much easier than setting it manually.
BTW, tmobile prepaid is really hard to get google voice to work, I'm sure it's doable, but I've read many people say it's not possible.
I just ported my service from ATT about 2 weeks ago, and it works just fine.