Credit added with my account unauthorized

The first thing you should do is to at least change your IMDb password, so that whomever it was who did it can't do it again.

It appears that somebody has been utilizing your account. If you logged in to IMDb on a public computer, but did not log out, any other person who used that computer would be able to enter IMDb as if they were you.

Alternatively, if someone other than you has access to your computer, and you do not explicitly log out of IMDb when you leave, then they will have access to your IMDb account.

The best thing for you to do is to change your IMDb password immediately. While this won't help if someone is using your computer, it will prevent further use of your account if you stayed logged in on a public computer or if someone has actually obtained your current password.