I started using Emacs and I wrote a text in Greek. The problem is that the stress that is used upon some letters (ό, ύ, ί, ύ) doesn't work!

It is usually inserted by pressing the ; (I mean by pressing the key next to L) and then the letter upon which will be inserted. When I press the ; it says:

<dead-acute> is undefined. 

What can I do to be able to use this key?