Please read the
Rockbox Manual.
You do not need to sync anything through WMP. Anything you do sync will only show up in the Sansa's original firmware. Rockbox supports MSC (like a flash drive), so to transfer music you just have to drag and drop:
Turn on the Sansa; it will boot into Rockbox.
Plug the Sansa into your USB port on your computer.
Wait for the Sansa to show up and be assigned a drive letter.
- If you have an SD card inserted, it will get its own drive letter
Drag your music files onto the Sansa or SD card as you desire.
Unmount and unplug when done.
Your files are browseable in Rockbox through the Files menu as a filetree (similar to Windows Explorer). If you prefer browsing by Artist, Album, etc., then use the Database menu.
And again, please do read the manual. Many people put a lot of work into it, and it explains everything you'll ever want to know about Rockbox.