So you're trying to serve your beer from the pressure barrel, not bottling? Without some means of co2 being generated or forced in (via the co2 injector) in the barrel, or priming and bottling your beer it will be flat.
The easiest thing would be just to go get that injector and follow the instructions.
There should have also been instruction on how to naturally carb in there or at least how to properly bottle. You have to do something, fermenting beer, although it has some co2 in it is far from carbonated. There's extra steps involved as you can now see.
And it's lager, not larger.... Larger is bigger