Sounds like your phone may not be properly connected to your PC. Try these steps:
1. Install Motorola Phone Tools (MPT). Do NOT plug in your phone yet.
2. Update the software to the latest 4.5.1c.
3. Plug in your phone using the USB cable.
4. Allow the PC to detect and install the correct drivers.
5. Launch MPT when complete.
6. Follow instructions to connect the phone to your PC.
7. Use the Media Studio to transfer the music files. It will convert to the right size and format recognizable by the phone.
If you have still have problems, download the DriverTool. You can download it here:
http://www.bvrpforums.com/motorola/viewtopic.php?t=2935
What this does is to clear out the older drivers that may have been incorrectly installed. So you'll get a fresh start.