Vampires / Werewolves

[bambigoddess]

This is an answer taken from Yahoo:

You need to make friends with a vampire and get them to bite you. After that, that sim can turn anyone you choose into a vampire between the ages of teen-elder, you can't turn children, toddlers, or babies into vampires. To get a werewolf, wait until night and a werewolf might come, to increase your chances, put trees all over the lot and if you have a child or younger on the lot they're more likely to appear (Warning when the werewolf comes, your game will slow down a bit for a minute or so). Make friends and the werewolf will bite you, after that, just like the vampire, that sim can now turn any other sims into a werewolf.

Or you can download a mod/hack and get them turned into a vampire or werewolf by using it.

If that doesn't help, let me know and I'll see if I can explain it any better.