I realize this has been answered, but I can't get it to work. I have a new Google account, and I want to associate it with my existing Stack Overflow account. When I log in using "add opened", it just logs me in to a new account with 1 reputation, and it doesn't migrate the new account to my old (current) profile and information. What am I doing wrong here? It is the same when I try Facebook.
According to Jeff's blog post about this, you need to email firstname.lastname@example.org with either the user IDs or the User page URLs to be merged and they will combine them for you.