What Wongo said. As a Green member, I've also been able to select specific seats (including exit row seats) when doing online booking. You just need to enter the MPC number as you proceed to the payments page.

Note that if you enter another FFP's number at the payments page, you'll subsequently lose the ability to see your booking in the 'Personal Itineraries' section of your Marco Polo Club account page.

And welcome to FlyerTalk, rowie747!