If you are using MP3Tag, which is awesome BTW, but don't see the changes you make to the tags outside of the program such as in Windows 7, ITunes, or even Windows Media Player then the problem is probably not MP3Tag, it's likely the MP3 file is somehow corrupted.
Spent quite a bit of time updating MP3 tags using this program and thought I had cleaned everything up. However then I noticed that several of my songs were not visible in WMP or ITunes. I also noticed that all the tags that I had spent all that time putting in were note showing in windows explorer. I tried to edit them using the file properties in windows explorer and it wouldn't let me. I went back into MP3TAG and surprisingly the tags were still showing correctly there.
After doing quite a bit of reading, I found a utility called MP3VAL that is basically a scanner/repair utility for MP3's. It scans the files and identifies which have problems. Not surprisingly all the ones that wouldn't show in ITunes/WMP and wouldn't show the Tags in Explorer all had problems. However, several others had problems as well. I then ran the repair function in MP3VAL and, presto, all the files now show as they should and my previously entered tags even showed up.
The link to the utility is below. Hopefully this will save some other newbie a lot of frustration that I went through.
Select the first option which includes the frontend and core engine. Unfortunately there is no installer but cnce downloaded, simply unzip it and copy to your Programs directory in a folder you create. Then just create a shortcut to the frontend executable on your desktop if you like.