I have spent all day Googling and reading posts where folks get stuck on this one. The MP3TAG help index only discusses freedb under the sources topic.
There is no obvious way to enter my Discogs token or credentials in MP3TAG.
Can I get a step by step set of instructions to get Discogs working as a source in MP3TAG?
Another issue could be, that there is no default browser associated with URLs. Can you try "Help > Mp3tag website..." and see if it opens a browser window and the website?
This means that MP3tag and its scripts do not have internet access, probably due to being blocked in the firewall or something.
You would have to adapt your network configuration.