The Westin's parking garage is located underneath hotel. The entrance is next to the hotel's main entrance (revolving door).
To get to the $5 (must exit by 3am) parking lots leave the hotel and turn right onto Summer Street, at the next light, turn left on to D Street, go over the bridge and turn right onto Congress street, immediately after the building at the corner, turn right behind the building and then turn left into the parking lot. You'll need a credit card to enter the garage, and then the same credit card to exit the lot.
More directions to come