If you have used a differnt OS before (other than W8) and updated to W8 it could be that you are not the owner of the folders where the settings are stored.
Usually they are stored in %appdata%\Mp3tag.
Also, you should install MP3tag with the same user account that should do the editing afterwards.
If you want to claim ownership of a folder, and you do not know how, check the microsoft pages
http://support.microsoft.com/kb/980023/de