Easy, Download it just like any file, Un-zip it and look inside the file 1. It has a file named GameData that you can place in the Ksp_osx file named GameData 2. It has multiple files named Parts,Plugins,and Ship, Simply open one file (easier to start with "Parts") and open the Ksp_osx folder and open the parts file inside. Next put all stuff from the mod file and place it in the Parts file (Its the same with all the other files, i think)