Gentleman: You can wear a tuxedo, but it's certainly not a requirement. Ladies: Feel free to go the long evening gown route, but a dressy suit or cocktail dress is fine too. Just wear something that makes you feel like dancing!
If your legs are as long as Jonathan's, it will take 200 steps. Turn right out of the Meeting House onto Washington Street, then a left on School Street. You can't miss it.
If you're a public transit fan: Green Line to Park Street or Orange Line to Downtown Crossing. If you're more of an instant-gratification type: Uber/Lyft. If you really want to drive your car, Pi Alley garage is your best bet.