Some general advice for travelling in Warsaw can be found on wikitravel
See the route map on Google Maps.
Start address: Zwirki i Wigury
End address: Grochowska
Start at: Zwirki i Wigury
After leaving the airport terminal 2, go straight ahead to the bus stop. Bus number 175 will take you from the airport to the City Center area. Leave the bus 175 at the stop Dworzec Centralny, and take bus number 521 (this bus will also take you to the conference place) and leave it on the Plac Szembeka bus stop. The hotel is walking distance from that place.
Alternative route:
Bus number 188 will take you to the Wiatraczna roundabout.
After leaving the bus on the Wiatraczna stop, you can go to
the hotel with a trams numbers:1, 3, 6, 24.
You should leave the tram at the Plac Szembeka tram stop. You will be
walking distance from the Felix hotel.
The route in this case is reversed, the starting place is Hotel Felix (Omulewska Street), stop place is Plac Szembeka bus and tram stop (Zamieniecka Street). Using the map one can choose the route for himself - its giving directions for drivers, pedestrains can choose many more paths. To get from the hotel Felix to the Conference place, one must to go on the Plac Szembeka bus stop and take a bus number 521, then leave it on the bus stop Szachowa. The Institute is walking distance from there.
The address and a small map of the conference venue is given on the NIT website:
National Institute of Telecommunications
Szachowa Str. 1
04-894 Warsaw
GPS: LAT: 52° 10' 21,7'' N; LON: 21° 11' 41,4'' E