Ganito yun... if you intend to add drums without a drum set in sight, you will have to add it using midi. I'm not sure if Cubase LE comes with the Unversal Sound Module, just search the steinberg directories for usm.dll. This is a GM-compatible software synthesizer that usually comes with Cubase. In a pinch, you can also use the Microsoft GS synth that comes with windows pero you will have to make sure that midi output is active in Cubase's Device Options, I think.
You will have to know something about midi to pull this off though. If you know a little something about Reason or Fruity Loops, you can create some rudimentary drum tracks at first, possibly import them into Cubase or even use Rewire, but we're getting ahead of ourselves here.