I've just downloaded the program, but when I try to view my audio files in the folders where I know they reside, they don't show up. This is true of all the audio files, in any format.
Unfortunately, you don't say, where they don't show up.
If you select a folder with the standard windows dialogues, you only see folders but no files.
If you do not see any files in mp3tag although the status bar tells you that you loaded files, you might have activated a filter. Press F3 once to close the filter and check if you see the files now.