Is there any way I can put in a certain sound.wav in a track each time it crosses a given limit? Like a drum trigger, but with a wav. sample instead of MIDI. I want to use that for kick drums.
Nope it's for both. REAPER has it's own native plugin scripting language called JS.
I'm not at my desk right now so this is going to be a little vague, but in the Options menu there should be something along the lines of "open resource path in explorer/finder." That will open REAPER's settings folder. There should be a folder called "plugins" or "FX" or something similar. That's where you install the plugin.
I believe there are installation instructions included in the zip file that you downloaded. Usually you just copy and paste a text file (that's all the plugin really is..) but this plugin also includes some free drum samples that need to go in the Data folder.
Once it's installed, open up an FX browser and refresh your plugins list. It should show up under JS.
I still have about 5 hours of day-job work today, but when I get home I can post some better instructions for you.