The Drum Map manager just basically lets you tweak a note-drumsound mapping that a MIDI synth uses. Let's say for example that you record a MIDI drum part and you use a GM (General MIDI) keyboard drum patch to produce the drum sounds and everything's sounding cool.
But say that you decide later that those drums don't really sound quite right, so you're going to try out a softsynth or soundfont or some other MIDI synth where those MIDI notes don't produce the same hits. (snare hit on the original is now sounding as a cowbell and stuff like that - could be just a few notes are off, could be a bunch) So, instead of going through and moving all your MIDI notes, you could just load up (or create) your MIDI-synth-#2 drum map and send the Out of your track to it.
That's one use for it. It helps with other stuff too, like it shows you the drumhit name in the Piano Roll view. Also, say you are just going to be using a basic kit's hits, not the extra stuff like bongos and whistles and all that crap, then you can make a Drum Map with just those notes defined, so then when you go into the Piano Roll view, you'll just see those.
Anyway, you don't NEED a drum map, but in some cases, you might find that it makes life easier. So, that being said, if you just want a basic kick/snare/hhat track, you can just... go to a MIDI track, assign the Channel to 10 (most likely - can go to Options->Instruments to see how your channels are mapped), make sure that the Out is something that will produce sounds (most higher-end soundcards don't have onboard MIDI synths even though they may have MIDI In/Out) like a soundcard's synth, a softsynth like the Edirol VSC DXi, or an external MIDI synth. Draw in a basic beat - copy and paste 'til your heart's content.
-Jeff