Welcome to LMMS forum!
read
https://lmms.io/wiki/index.php?title=Working_with_VSTs
That covers vst-instruments, and is what you ask for
This is for effects
https://lmms.io/wiki/index.php?title=Wo ... Ts_effects
but not related directly to your need for a percussion VST
(Unfortunately, i have to tell you that MAC does not support VST. Hope that does not apply to you)
The way you actually use drum machines is to set up the loop,then use LMMS-automation-tracks to start stop and change different loops. Can be tricky, but definitely possible.
Just ask for more, if you need more info. If you are new to LMMS i recommend the whole wiki, and (ofcause) my youtube-channel, where you find a lot of tutorials :p (signature)