Here's what the Prima Guide says about the new gnomes:
The Celebrity magic gnome is sometimes found in the catacombs below the graveyard or inside ore and uncut rocks. This magic gnome loves to be placed near hot tubs, professional bars (on lots you own), and expensive vehicles. The Vampire magic gnome, on the other hand, only shows up if you have several Plasma Fruit in your garden. The exact required number changes all the time, so if you really want to see this gnome, make sure you have plenty of Plasma Fruits on your lot. The Vampire gnome is an adorable little fellow -- and if you happen to have a teddy bear on your lot, it loves to sneak up on the bear. The gnome may even turn the teddy bear into a vampire gnome!
This is pretty different from what I've seen posted on various forums.