I believe your issue was from using a T-Mobile SIM Card. You are supposed to use a NON T-Mobile SIM Card and punch the unlock code in the dialer..
STEP 6 of the OP:
You can now use the unlock code when you put in the non-T-Mobile sim into your phone and unlock your phone. You can check the unlock status by dialling *#7465625#. All the locks should say "OFF".