From the MusicBrainz iTunes Guide:
Updating Library in iTunes
Brute force method (full library re-scan)
This method works best if
- you've re-tagged a whole lot of your files and don't remember which files/albums
- or there are far too many for the first approach
- or you've moved/renamed files and iTunes has lost them
- Go to File > Add Folder to Library
- Point it at the top of your iTunes directory ("My Music\iTunes\iTunes Music\Music" on Windows, by default).
- iTunes will rescan your music, but recognize it's already imported, so you shouldn't end up with duplicate files (unless you have moved/renamed files)
You can also refresh a selection of songs by following the AppleScript based solution (on the same page linked above):
Mac-only advanced method
This method is a bit more complicated to set up, but easy to use.
Fire up Script Editor (which is under Applications/AppleScript, but it will work with TextEdit). Paste the following code, then save as a Script called /Library/iTunes/Scripts/Refresh.scpt (create the Scripts directory if it doesn't already exist).
tell application "iTunes"
Now, inside iTunes, you should see a Scripts menu (an icon of a scroll, just left of Help). The "Refresh" item on that menu will force it to re-read the tags for all tracks that you have selected.