It's pretty simple, open a folder within mp3tag and it will show you the tags for all of the files within that folder (and subfolders).
Since youre getting 'unknown artist' etc you will probably find that most of your files are untagged, so you will have to add the tags yourself. The most important tags are:
title (the name of the track)
track (the track number)
You can either edit fields individually (or select a bunch of files by, say, the same artist and rename them all at once), some help on this here: Tags - Mp3tag Forums
OR you can use the 'convert filenames to tags' option, which is much quicker. Help on this here: Converter - Mp3tag Forums
OR you can have mp3tag go off to a central database and figure out what tracks you have and tag them automatically (i've never really tried this but apparently its very good)