The problem seems to lie in creator types. The files LimeWire is saving to your hard disk don't have the right creator codes, or whatever, so Toaster doesn't think they are real mp3s. So, I would recommend finding a program that can change file creator codes ("filebuddy?"). I do own a mac, so I cannot offer much more help.

Do you have iTunes installed? New versions of LimeWire save mp3s as iTunes files. Perhaps you could use iTunes to burn them?