Take the KTH exit from Tekniska hogskolan T-bana.
At the exit of the Tbana turn to the left
Then go up the hill straight past 711 and you will find an intersection
PIC.
Turn to the right and go about 50 meters until you see 2 columns with three-headed dogs on the top.
(It is behind the red car in the picture)
Go through this large gateway
Then, take the first door to the right (I think D2 is marked on the outside of the building).
Within the building walk straight ahead and turn to the left at the end of the corridor
You should see a sign with D2.
I think there are signs within the building.
You can ask in the KTH info center as well.