Here is some documentation from Discogs about how an application (like Mp3tag) should ask users to authorize the app on behalf of their Discogs account.
It seems to say that Mp3tag should redirect users to the "Authorization page" (presumably a page like this), before they are asked for an Authorization code.
If the OP is not being re-directed, then there may be a problem with Mp3tag or at least the user's particular installation.
What can users expect to see on screen when first attempting to access Discogs?
Will they be re-directed to an "Authorization page"?
It would be helpful to have a step-by-step guide for users to allow and/or repair access to Discogs, even if some of the steps involve the Discogs website.