I was having the same problem on my Curve. Could not get Roxio or Media Player to sync to my phone correctly. I ended up dragging the files over to my phone in Windows Explorer. Picked up the folders I wanted in "My Music" and copied them to my F: drive (where my SD card shows up on my laptop).

I don't really use playlists, just listen to complete albums. So this worked well for me. Not sure how to create playlists that sync.