I've put up quite a few posts and I'm getting no where as well. Most of the guys seem to say that you have to do a few things to it to be recognized as a "modem". I've gone and downloaded VCast w/ Rhapsody and went and looked of USB drivers as well as driver that will help with this problem but I too am having no luck.
I got the phone around July 4th and I got the USB cord for Christmas and I've been trying since then to get just one song from my computer to my phone. The only thing holding me back is that when I hook it all up, I'm not having that chime that you get when the computer sees a USB being plugged in. I think that if I can get that chime to sound, I might have something but no luck. I even tried it on another computer and it's the same thing.....no chime.
One option I've been thinking of is to try and find a way to get the music on the SDmicro through the SD adapter and just put songs on it in a format that the phone will see but I don't think that can happen.
The only thing that I know is that if you have it on and you put in *#6870+end it'll disable the MTP (whatever that is) and you should be able to put music on it. Once the download is done then you push in *#6871+end and it'll put the MTP back on. Everywhere I go that's what everyone is saying but I've done it and it's still wont be seen by my PC, not even in the Rhapsody source list on the left. It's not even showing up on the device manager nor my "My Computer" window as a "removable storage". I think it might be the same reason that I can't get my computer to link up to my 360 to play music and vids.
If you found out something let me know, PLEASE!!