Where did you get the unlocked iPhone 5 from? Been waiting for over two months for it. Did you get it shipped from Canada?
You can buy it at the Apple Store, if you tell them you want to buy it at full retail, no contract. For T-Mobile, make sure you request the AT&T (ugh) model, so that you get the right LTE bands for the LTE rollout next year. (You can probably even just flat-out say you want to use it on T-Mobile. I get the sense that they don't really care.)
Provided you pay the full retail ($649/$749/$849), their system will automatically register the phone with Apple's IMEI database, and when you get home and connect it to the iTunes, the first thing you'll see is those beautiful words: "Congratulations, your iPhone is now unlocked."
I've done it several times now. Worked beautifully every time.