On Casual mode you can just take the elevator right up at some point.
On Hard mode, as the ghost character you need to collect the cake from the Thimblecon convention, chill it and give it to Clara. Keep talking to her and you'll find out what show she likes to watch on the guests TV. Tune the TV to that channel on Floor 10, and zap the door, she'll come out, and you can get your guest to the penthouse.