I had the same experience you did - heard everywhere the SGS2 had a tuner, thought that was great...then, I bought one (I'm a TMo customer, like you), took it home...then realized I couldn't make it work.
Popped a message onto the TMobile support forums, and the answer from them at the moment is that the FM tuner *is physically not present* on the TMobile SGS2. Looks like AT&T has done the same thing for theirs.
Now, I don't know if the "physically not present" can be trusted entirely, because when I tried a couple of different radio apps, both detected when I had a headset plugged in (for the antenna). Maybe that's a coincidence, maybe not, don't know. My understanding is that the FM tuner incorporated into the Broadcom chipset uses some sort of proprietary instruction set, and I *think* that was the chipset TMo had to use to let the SGS2 talk to their 42Mbps network (which, in turn, led them to replace the Exynos CPU with the Snapdragon).
If the FM tuner is there, its apparently going to require a custom ROM or a formal update to turn it on, and I frankly wasn't willing to wait on Samsung to release an update based on their relatively poor support history on such things. I returned the SGS2 and picked up an Amaze, and it works perfectly. The Amaze isn't quite as sexy looking as the SGS2, but I needed the FM tuner more
Hope that helps.