First, please click on the message chute, then on the letter marked with a star. If you haven't confirmed your email-address already, please do so via your Profile. In this case you should be seeing a respective alert in your message-area.
If you already confirmed your email-address, then enter the player you would like to contact, a subject title and text now and send off your messages.
Please note, that you can only contact players who are registered on the same hospital ward (game world/server) as you are.