Take the red, orange or blue line train to Metro Center. Exit onto 12th St. and head north (toward the Washington Marriott). Walk up several blocks passing H Street and New York Avenue. The building is on the corner of I Street and 12th, across from Bobby Van's Grill.
You can also reach CASE by taking the blue line or orange line train to McPherson Square. Exit onto 14th St., going west onto I Street. Walk two blocks until you get to the corner of I Street and 12th Street. 1201 I Street is the building on corner, across from Bobby Van's Grill.