Basically:
Download both files
Unzip the master file and you get a "data" folder that needs to go inside the "coh" folder that should be in your Applications folder
Unzip the add-on file
Drag the contents of the new "data" folder inside of the "coh" folder
I forget the verbiage, but you need it to overwrite the existing files with those that the add-on contains
I remember having an issue during my test with this because unlike windows, it unzips directly into the folder I was in. So you may wish to make a new folder inside wherever you unzipped the original first, drag the add-on zip into it, and unzip into That folder so that it doesn't rename the data folder it makes. Then you can drag the data folder from the child folder into the root and it 'should' ask to overwrite the files. At that point you can drag the combined "data" folder into the "coh" folder. I keep meaning to do a write-up, but I have about 10x the life stuff to work on right now than before spring.