[MunchkinMan]

You first have two choices with respect to how to use the card, one being to add a new Monster, the other to move one already in the Dungeon, correct? However, you are wondering about adding a new one, so you follow all the rules you would normally follow when adding a new Monster to the dungeon when you explore: Roll the Monster die to determine ownership. If that player has no free bases, choose another player, or, if there are no free bases at all, remove the highest level Monster from the dungeon (except the Boss Monster, if this is the final battle at the Entrance) and use its base.

For reference, the cardboard cutouts of the Monsters are the standies, and the plastic things they go into are called bases. Every Monster standie should be in a base, and you should only be using bases that are associated with active players.