What video editing software are you using?
If you purchased the song from Napster I'm guessing it's in WMA format w/ DRM/copy protection included.
If you want to use the track in your vid you'll need to convert the WMA song to a format that both your vid software can use (like WAV or possibly MP3) without the DRM information. Do a Google search on converting Napster WMA files to MP3 or WAV and you'll find plenty of information.
Usual disclaimers apply-- I'm not advocating music piracy or anything. You can be the judge of reasonable use of the song and for personal wakeboarding vids I'm guessing the RIAA isn't coming to serve you a lawsuit.