Obvious question: Did you actually set the game to advertise on the MS?
Not so obvious query: Is the MS even up when you host? Sometimes the MS itself shuts down briefely while I'm playing (sometimes before I start a game, even) - if you use the listserv command in the console and nothing shows up, that may be what your problem is.Beyond that, well, I'm not really sure what to say yet. I'm assuming you can still join games you didn't personally host, right?