help with the presidential metro(possible spoilers)

Where on earth is the presidential metro? been walking around the metro for time now, my marker is telling me to to go to the Pennsylvanian exit but this just takes me outside.
There appeares to be some kind of tunnel system underneath the said exit but i'll be dammed if i can find it. I may be missing something rather obvious but im ill from work today which may be affecting my ability to find it and thus this may be the most retarded post on here for some time but any help will be most appreciated. Please help its dark and lonely down here.....

much love