When you receive your welcome pack from BMW, it will contain plenty of information on getting around in Munich, with good directions for getting to the Welt via public transportation. It's very simple, as you will see.
As for your flight home, unless there's a compelling reason to visit Frankfort, I would suggest that you drop off your car at the Munich airport and fly home to Philadelphia from MUC.
Prod Date 4/5/12, ED: 4/19/12, Redelivery 6/20/12, 535xi, TiAg on Gray Dakota, Anthracite Wood, Cold Weather, Premium Pkge, Tech Pkge, Side and Top Cameras
ED: 2007 530xi, Monaco Blue on Gray Dakota