Greek letter shortcuts in Keynote 6.5
Using the Keyboard System Preference, in combination with the Greek keyboard, I was able to get this to work.

Open System Preferences > Keyboard

Select the Keyboard tab and check Show Keyboard &amp; Character Viewers in menu bar. The flag of your default language should now appear to the right of the Date/Time menu item.

Select the Input Sources tab > Add Greek keyboard

Select the Text tab > select then add \lambda to the Replace field

From the Character Viewer menu bar item, choose the Greek keyboard > Select the With field and type the 'l' key. This should insert the λ character in this field.



From the Character ViewSwitch back to the default keyboard.

Now, whenever you type \lambda you will see this replacement pop-up:



Repeat with other characters.