MIA takes place every year at Philly's iconic Benjamin Franklin Parkway, in the immediate area directly surrounding the Philadelphia art museum. (Rocky steps, yo!) The most convenient way to get there via public transit is into the Amtrak 30th Street station, which lands service from NY, DC, and more. From there it's just a 10-minute-or so walk. Pro-tip: use the John F. Kennedy bridge and hit the walking paths along the Schuylkill River.
If you're coming from Philly proper, the SEPTA train and bus system is fairly reliable, but can be a little tough to navigate for outsiders, and it doesn’t stop super close to the festival. Your best bet is to drive, bike (Philly's got a great bikeshare system), or catch a cab to/from your hotel.