how do i get the Greek alphabet on my mac keyboard?
These should help you; they are "stickied" to the top of the Greek forum:
Good luck (and welcome to Duolingo!)
At the top right corner of your Mac, you should see a flag. This shows the keyboard you are currently using. Click on it, and press "Open Keyboard Preferences". Next, go to "Input Sources' and press "+". Scroll down until you find the Greek keyboard and press "Add". When you want to use it, click on the flag again and select it from the list.