Drums machines are line level. As a general rule you shouldn't need a pre-amp with a drum machine.
How they hook up to a mixer depends on the drum machine. Many machines simply have stereo outs, but some machines may have 4 or more outs. Every machine I've ever seen has standard 1/4" outs.
So- if you have simple stereo outs connect two 1/4" cables from your drum machine to your mixer (1/4" line in), pan the mixer channels to the far left and far right. You can then assign stereo placement with the control parameters of your machine.
Normally, the kick and snare are panned center (12 o'clock), the cymbals are panned maybe 1 or 2 o'clock and 10 or 11 o'clock and the toms and percussion are panned 3 and 9 o'clock.
When doing stereo panning with drums, try to enviison looking at a drum set on the middle of a stage, and pan the individual drums/cymbals as you see them - the main thing is avoid extreme panning (unless you are going for a specific effect)
If you don't want to use two channels on your mixer (and if you don't care about stereo drums) you should be able to assign all the drum sounds to one out. In fact many machines will go to a mono default if only one out is connected.
Obviously if you have more than stereo outs, it all depends on how many mixer channels you want to tie up. With multiple outs, it is common to assign the kick and snare to their own outs (for maximum control) and then use the other outs for stereo toms, etc.
Hope this helps!