Hi, Welcome to the Forum lfrisancho1 !
Here are all important links:
http://lmms.io/forum/viewtopic.php?f=1&t=4740
-A few rules and useful forum instructions
Not possible im afraid, because most drumkits are VST-build, and MAC do not support VST -However!I've seen several tutorials on downloading drum kits to PC but none to Mac. so not sure if its even possible
If you download Hydrogen, all the percussion-sounds are freely accessible as single sound-files. Tose you can simply just copy to your collection of samples, and they will load directly in Audiofileprocessor, so you can add new percussion to MAC-lmms, that way. There may be other drumkits, that you can use the same way