Look at the Tagging window, at the upper left. See the star shaped icon? Click on it and select "customize". This is how you customize the tagging window. Notice that at the top it has a "profile" selected. There is a profile for each media type. You should customize the one for the type you are editing. So if it is music, customize the Audio profile. It should be auto selected if you were looking at the file type you are interested in.
You can add your own fields wherever you want them, remove the ones that are there, etc. I think you'll get the hang of it pretty quickly.
Brian.