how do you add greek letters?
I expected the greek letters to be listed to click on, as for the accented letters in italian, french, german etc.
Since the Greek alphabet is its own completely separate alphabet and it isn't just a few additional letters with accents and umlauts, it does have its own keyboard. What OS do you run?
If you do one of the first few Greek lessons using your web browser (not the app), the Hints and Tips will tell you how to add the Greek keyboard to PCs and Macs ... on mobile devices it's in the settings.
(And it's always worth looking at the Hints and Tips for all the useful information about that particular lesson anyway ... even if you only do it once for each lesson!)