The best way is to take the Metro.
Take line B (the yellow line) and go to the Náměstí republiky station.
At the station follow the navigation in direction Náměstí republiky/Paladium and take the escalator in that direction.
Then, in the hall take the escalator on the left.
Above the escalator exit there is a white building . Go straight to the right side of the building and in 50 meters you are in the theater.