Go to the BCS download page and click the Download BCS App under the Mac section.
The app ZIP file will download and usually appear in your browsers download bar. The image below is from Chrome.
Click the arrow next to the download and choose View in Finder.
Double left click the ZIP file. This will unzip the app and it'll appear in your Downloads folder as MacBCS.app
Double left click the MacBCS.app file. This will popup a warning dialog about downloading apps outside of the official app store.
Click the Okay button to proceed and the app will open.
That's it, the BCS is now running and testing should be possible. To ensure the BCS is running correctly click here. If all is well a green Successful message should appear on the page.