Demtech can be found on the fourth floor of the IT University of Copenhagen (namely, in section 4C). ITU is located at Rued Langgaards vej 7, on the island of Amager in Copenhagen.
The easiest way to get to ITU is to take the Metro to DR Byen, and walk 400m north on the paved footpath. You can reach Metro station DR Byen,
from central Copenhagen: by getting on a Metro train towards Vestamager;
from railway station Ørestad (between Copenhagen Central Station and Copenhagen Airport): by getting on a Metro train towards Vanløse;
from Copenhagen Airport: by getting on a Metro train, and at Christianshavn, switching to the line towards Vestamager (across the platform).
Other routes, tickets, etc. are available from Rejseplanen.dk.