1. Go to your ableton plugins

2. Create a MIDI track

3. drop the simpler (for a one sound sampler) onto a track or the Impulse (for multiple sounds)

4. drag and drop your percussion samples on-top of these devices.

5. Go to your midi track in the arrange window and double click in a slot to create a part. (it should appear like your audio parts usually do)

6. you can then use your mouse there and create MIDI parts

Best to visit www.ableton.com and join the forums, you dont have to be a registerd user and also there are tutorial videos

try youtube as well loads of tut vidz there.