Not to be confused with the discussion space of a notebook to which all members of the notebook have access. 

From your Homepage, you have two ways to send a private message to a contact.

Méthode 1 : from the left side menu

By selecting the Contacts tab and then the desired contact:

Méthode 2 : from the navigation bar

You can send a new message by clicking on the button  on the navigation bar, at the very top of you homepage :