Hi SurfRat!!
As Steven and Zak have already told you, AAC files are raw audio stream data and haven't any ability to be tagged like MP3 files.
If you want to write tags to AAC files you first have to move such files into a mp4 container using e. g. mp4box (command line utility) or if you want a GUI for that then use e. g. My MP4BoxGUI. By moving each AAC file into a mp4 container the file gets what is called atoms (comparable with the mp3 tags). And now you have the ability to tag those files: writing Artist, Title, Composer, Artwork, etc. into them.
BTW: By moving those AAC files into a mp4 container the file extension has to be changed to mp4 or, if you prefer, m4a.