The way to get a Greek letter on the Mac is:
Go to Systems Preferences> Languages & Regions > Keyboard Preferences
click on the + sign bottom left and select Greek from the drop down menu.
Also select Show input menu in menu bar
Quit Systems Preferences.
Now in the menu bar, by the date will be a flag indicating the language used.
Select Greek and Show Keyboard Viewer.
This will show the keyboard with Greek letters.
Select the letter one needs.
Return to the flag and select the flag of the country of your usual language.