I'll answer for Reaper since that is what I use.
The LPD8 sends out MIDI controller messages. The first thing you need to do is to ensure that your DAW is receiving those messages. In Reaper, go to preferences and make sure the LPD8 is enabled as a MIDI in device.
To use it to control drums, select Track | Insert Virtual Instrument on New Track... and select your drum sampler plugin. For example, I have selected ToonTrack EZ-Drummer here:
The important things to notice compared to simply adding an empty track are that it has set up four things for you:
- the drum sampler has been added to the fx bin
- the track has been armed for record
- MIDI All devices and all channels has been set for input, so it will receive the MIDI from your LPD8
- Record Monitoring has been turned on (so you will hear the noises made by the drum sampler)
The next step is to ensure that your LPD8 is sending appropriate messages when you hit the drum pads. It can be configured to send many different message types, so you might need to refer to the manual and use the supplied software, but I strongly suspect that there will be a factory bank that sends MIDI note on messages for the most common note numbers that drum samplers are mapped to (e.g. kick drum = 36, snare = 38, closed hats = 42 etc)
The second part of your question, to use the knobs to change parameters, you need to investigate a feature called MIDI Learn (which most DAWs support). This lets you tell the DAW which controller on your keyboard you want to use to control a specific parameter. Here's a tutorial on using MIDI learn with Reaper.