I recently bought an iPhone 3gs and I can't get it to unlock with iTunes?

I put my T-Mobile sim card in the phone (the phone is unlocked for tmobile) and i connect it to iTunes and it says "The SIM card in this iPhone does not appear to be supported" why is this happening? Should I just give up on it and take it to the apple or T-Mobile store?

You can go here: http://www.unlockcellphonecode.com/ and then repeat those steps.