Actually, if you have a premium membership, you can simply enter you login details by going to Tools - > Options... -> MMOUI WoW (tab). As soon as you enter your (correct) username and password and your account has been validated as "premium", you can simply "Close" that window and the ad window inside Minion will disappear.
More information about Premium Membership can be found
here and yes it is true that you will require an Allakhazam account, as WoWI is part of the
Zam Network and premium applies to all of the "children" websites (
Premium FAQ). You are not limited or required to use the other websites, in any way. As for the problem you are experiencing, pming one of the site admins (Cairenn or Dolby) would be the correct course of action, so hopefully it can be addressed as soon as humanly possible.