Open Question: How do I put downloaded music files on my Onyx MP3 player?

Here's my problem:

When I connect the Onyx MP3 to my laptop computer, it looks like it connects because the menu comes up, but when I click on 'Add Files', it says 'the trial version of this product has expired'. Also, there seems to be no other way to drag music I have downloaded into the Windows Media Player to synch it, because Windows Media Player just doesn't come up at all. What exactly is the problem here? Has anyone else had this problem?