If you want to reach me via G+ messaging, you need to be part of my extended circles. That is, we need to be connected on G+ by no more than one degree of separation. I need to be able to see your G+ stuff and my flow must be visible to you if you want to use private messaging in Google Plus. I turned on the private messaging feature by editing my G+ profile after going to http://gplus.to/topgold and clicking "Edit Profile" on the screen. That exposed an area just under my G+ Profile Image (in the screenshot) and I changed the message settings there. Those settings let me see messages only from selected people or from specific circles.