I'm assuming you've tried dragging the mp3s directly into the itunes window or selecting Add to Library from the file menu and choosing the folders with mp3s in the file selector.

If that doesn't work, it probably means that the files are either corrupt or have the wrong file header. The latter problem can be sorted with itunes Associator which will fix the header to work with itunes (drag the mp3 over the itunes Associator icon). Be careful with this program as it will do the same to ANY file whether it's mp3 or not! If the file is a valid mp3, you should then be able to drag it into the itunes library window.

If you want to be able to double click the mp3 and open it in itunes then go to the apple menu=>Control Panels=>File Exchange, find mp3 in the PC Suffix list and change the host application to itunes (click change and find itunes in the file selector).

If none of this works, it means you've downloaded dodgy mp3s.