The Math Fest takes place in Sollidenhuset at Skansen, Stockholm. The address is Skansen Djurgårdsslätten 49-51.
To get there using public transportation, you take the tram (no. 7) or the bus (67) and get off at the stop Skansen. From Stockholm Central Station, the easiest option is the tram. The stop is right in front of the correct entrance to Skansen. When you arrive at the entrance to Skansen, go to the special entrance to the left of the ordinary entrance and say that you are attending the math conference organized by KTH and Stockholm University.
Map
Special entrance
Venue entrance