Whether your troubleshooting a problem after updating your operating system or reinstalling a game on your new Mac, here's how it's done:
MOJAVE (macOS 10.14)
1. Sign into your Mac App Store account, then click on that account:
2. Click on the 'download' icon to reinstall your existing purchase!
High Sierra (10.13) and previous macOS versions
1. Sign into your Mac App Store account
2. Reinstall the game from the Purchased tab