Emacs Ocaml-Mode - HELP with eval phrase!

[Akil]

Hello there!
I've just installed Ubuntu 12.04 on Virtualbox and got the emacs ocaml-mode. 
When I open it I see the main page with the various information (emacs 23 and 
so on...). The problem is: I'm following my teacher's advice trying to write a 
simple expression in order to evaluate it, but when I write the expr and go on 
ocaml->"eval phrase" the Eval Phrase command is in grey and so it's impossible 
to click on it. 

Where I'm doing wrong? 
I'm sure that is a stupid thing, but being ignorant in this subject I can't see 
it...