From the comment above you, that you might not have been able to see due to posting timing: http://www.wikihow.com/Change-the-Keyboard-Language-of-a-Mac
You'll need to access the Greek keyboard on your computer. All computers have a variety of languages available, here are some ways to use them.
HOW TO ADD THE GREEK ALPHABET TO YOUR KEYBOARD Here are some links to easily convert your keyboard to enable you to use both Latin and Greek characters as you need. We have found these easy to install and simple to use.
For Greek alphabet with Latin letters: If you have a problem with your Greek keyboard and want to transliterate a read the :Greeklish: instructions on the forum. https://www.duolingo.com/topic/936
Don't forget to read the TipsNotes there is more information. Use the hover drop down hints. And of course, ask us if you have any other questions. Best of luck.