Once you've unlocked both the Vampire (Dracula) and Lord Vampyre's Bride, you can get this achievement. If you don't have Dracula yet, here's the guide to unlocking him:
Load up your game and visit your Monsters DLC world. Go to these coordinates to find a castle tucked into the corner of the world:
Use a helicopter or climb up to the castle. The vampires obviously don't want guests, because there's no other way to reach it.
Once you're in the castle, move around on the first floor (nearby the female vampire npc). We need to go to a specific location. I tried to place the minifigs "in the castle" in various places, but the achievement didn't unlock until I went here:
Notice the hole in the floor not too far in front of my character. Drop down that hole and place both minifigs in the brightly lit room:
They are now "in the Monster's Castle." Achievement unlocked! I'd say they're happy together, but they seem to fight a lot...