Alumino - I think you used the walkway to Terminal B, if you enter that terminal (after you take that escalator up) and turn left - you would run into Terminal A. After crossing up and over the road between the two terminals, there are shuttles that take you over to Terminal C and D.
The signage, according to another poster in the Washington DC forum, along with some of the terminal identifications will change for 1/26 when they put the train into production for passengers.