iTunes won't sync music that I've downloaded outside of iTunes

I've downloaded some music tracks from a musican's website directly and wanted to add them to my iTunes library / my iPod however after clicking through the usual "Add Files to Library" or "Add Folder to Library", there is no progress bar displayed and searching for the track names shows nothing in my library. The files play perfectly fine though Windows Media Player but iTunes refuses to reconise them.

The files are encoded as MP3 and I'm using both the newest iPod touch (64 gb) and the latest version of iTunes.