The OSX version stores its default presets, grains etc. inside the application. If you right-click the ArtRage icon and select 'Show Package Contents' (or its equivalent) you will see a folder pop up with a 'Contents' folder in it. Double click that and open the 'Resources' folder inside it. Inside there you will find the same folders that exist in the Windows version next to the executable.
It's preferable to use the Application Support folder however. If you open the Stencil Picker and select 'Manage My Stencils' (or similar) from the menu at the top left of the picker that will open the stencils folder in the user data location. Navigate backwards from that folder and you will find a Resources folder probably in your 'Users/<your user name>/Library/Application Support/ArtRage 2' folder. In that resources folder you can put various things like stencils and canvas presets.