I've found the easiest way is to not interact with them next time they visit you, just make sure you're around them and eventually they should nibble you.
Is it a normal wolf or a wolf with the glowing eyes? If it's just a plain normal wolf it's not going to nibble you. And not only that but you won't get a "nibble" interaction pop up under your sim, the wolf with the glowing eyes will decide when to do that. Once your sim is a werewolf then he/she will have the option to nibble other sims.
