We have still no time for complete manual on integrating with another DB, but there are some points:
1) Obligate fields (fields keys) in database for miniBB are: username, user_id, user_regdate, password, email, activity, user_viewemail, user_sorttopics, language. If they are not in database, you must add them and define in DB scheme. Another fields define user profile, and it is not obligate to insert them (but you can redefine them in DB scheme, of course, if your table uses another names for ICQ or website fiedds, for example).
3) For best reasons, you should disable registrations, editing profiles on your board (setting off the corresp. option), also as password receiving.
'NO FORUMS YET?' error actually appears not because of your users database, but because there are no forums defined at all. Be sure you are using correct database where miniBB tables are installed.