First I would select the songs in question and go to the advanced menu and click 'get track names from cddb'. (which must be done while online)
if that doesn't work, then you will have to type in by hand. for the track names, it is easiest to select the track and press the little enter button to the right of your space bar. (or at least it is on my powerbook; dont confuse the enter key with the return key)
that method works well for track name. for artist name you can select all the tracks, ctrl-click and then go to 'get info'. if you type in the artist and album name, it will apply that to all tracks.