Take I-55 North and get off at the Potomac Street Exit (#206A). Turn left on Potomac and drive straight until you reach St. Broadway then turn left. Then turn right at the next street, Lemp Avenue. The building should be on the left just before Potomac Avenue. (Note: This Potomac Street has no access to the before-mentioned Potomac Street.)
Take I-55 South and get off at the Arsenal Street Exit (#207C). Turn right on Arsenal Street, but be in the left lane so you can turn left on to Lemp Avenue which the intersection is right next to the off ramp. Drive south on Lemp Avenue. Drive until you reach Potomac Street where the building should be the first building on the right at that intersection.
WeSTBOUND (I-64 or I-44)
If you are on either Interstate 64 (Highway 40) or Interstate 44, get off at the Jefferson Avenue exit. If you are on I-44, you can continue towards the I-44/I-55 interchange and turn south. I-64 drivers will not have this options since there isn't a ramp from I-64 South to the I-55 at the I-64/I-44/I-55 interchange downtown.
So for both interstates, it is best to say to get off at the Jefferson Avenue exit then drive south until you reach Cherokee Street, then turn left (east) onto Cherokee Street until you reach Lemp Avenue, then turn right (South) onto Lemp until you reach Potomac Street where it is the first building on the right.