Resolved Question: mp3 player songs show up on one computer, but not on another?

My nephew has an mp3 player (a really cheap one, a Nextar 1GB if it matters). So I plugged it into my HP desktop computer, then copied some songs onto it.

But now that HP desktop computer no longer works. So I plugged the mp3 player into my Acer laptop, to put more songs on it, and to copy a few off. But when I plug it in, when the window comes up, no songs are showing up! It looks as if the player is empty, but the songs I put on there before are still there, and it even says "369MB free of 989MB".

So why aren't the songs showing up on the laptop, and how can I fix it?