When you open the DMG, you find a folder "renpy-7.3.5-sdk".
Copy this folder (the folder, not just its contents) to your Applications Folder.
Close the DMG, eject it, you don't need it anymore.
In the "/Applications/renpy-7.3.5-sdk" folder:
For the first launch, it's better to use Right Clic->Open (to avoid some security troubles). After, you can use the double clic without problems. So do it on renpy.app.
It may take a little time to open, depending on your computer, the position of the stars and your patience..
Now, the first thing to do is to define your projects folder.
Then clic on Preferences (bottom right of the window).
Top left of the preference window: Projects folder, change it (You can use something like ~/Documents/RenPy or whatever you want). You're games will go there.
Select you language.
In my very personal case, the Editor is the System Editor, the default one, as my favorite editor is not listed and everything is UNselected but material render and the one below, show the file edition section.
Now you can click on Return to go back to the main window.
To add your projects, it's easy.
In your ~/Documents/RenPy folder, create your new game folder. Let's say "Big Brother, the renew of the Fork".
You have now a "~/Documents/RenPy/Big Brother, the renew of the Fork" folder.
In this one, just copy the "game" folder (the folder, not just its contents) of the game you DL. The "game" folder is enough. No need of anything else like the "lib" folder.
Finally you have a "~/Documents/RenPy/Big Brother, the renew of the Fork/game" folder.
Go back to your renpy SDK window, click on the little "refresh" near "Projects", your new content should appear.
PS: As said before, some games won't work, because of the different versions of RenPy. It's a "some", not a "nearly every", but it can happen.
PPS: The "~/Documents" folder is your Documents folder. "~/" means "/Users/yourName/".
