A digram is a tile that has two letters in one, like CH or TI. Not all boards have a digram so keep playing until you get a board with a digram. Find one word using the digram and this will pop mid game.
Edit: If you're not able to unlock this, keep in mind this is in beta and only open to US accounts. If your Live account is in another country you will likely not be able to unlock these achievements at this time.