Does Erik's ghost appear at all even if he's not added to the family? Or does the entire thing just stop at a certain point? What is the message you get in the upper right corner? There was a time when the game was first released that ghosts would get stuck inside the Science Center and needed to be reset in order to come out. I don't recall if the ghost had been added to the family, though. I also thought this bug has been fixed because I haven't seen or heard of it happening in quite awhile. Have you exited and restarted the game?