I've built all levels and completed all missions in the game and I've still haven't seen Yoda, R2-D2 nor Darth Vader, despite having unlocked them in the Bitizens Album. Why haven't they begun to appear in my elevator, yet?
If you've already bought them in the album, check the store and see if they're for sale for 1 Bux. If you pay for them there, they usually show up immediately in your elevator.
There is a related question on the site here about whether unlocking the characters makes them available in the game.