I had the same issue with MC 8 and 10.9.2. Its fixed for me by moving the white Mac 'task/finder bar' in the display arrangement section to the desired monitor to be used for full screen playback. 

This does work, but WARNING; it changes the default start-up screen for every other application you boot.  If you're like me and have separate window-sets for each app (AE, Digital Performer, Avid) you'll have to reposition everything.  Plus my QuicKeys macros are all screwed up because they function based on the position of the default menu-bar, which is now on another screen.

A lot of hoops to jump through for something that worked in version 5.0.3.

Has this been addressed?

I'm on mc 7.0.4.2, OS X 10.8.5 and cannot get full screen to work. I've tried a couple of the convoluted voodoo tricks and nothing is working.

We need full screen to work in order to screen share the cut long distance. Any ideas?

Trying again and when i first open up the Full Screen settings file it opens over my right monitor, where I want it, has numbers in all 4 fields, then when I click on "choose/select monitor" the numbers all go to 0. THEN... when I click on cancel and open up the FS settings again they are way up in the top left corner of my left monitor, mostly off the screen so I cannot grab them and pull them where I can actually see them and do anything. I force quit Avid after that so that they wouldn't remain stuck there.

!!!

Okay, so apparently Avid decides that if you have an external monitor/Nitris DX hardware you don't need to go full screen. After turning off the Nitris DX and going back into MC full screen works fine.

I'd rather be able to make that decision myself than have Avid dictate when I can and cannot have full screen mode. Now I'm in a situation where we have to turn off the systems, turn off the Nitris DX, reboot, go back into MC just to get into full screen mode so that we can do screen sharing with our shoot location.

What a pain!

Does anyone have an easier solution?

Can't you use the little hardware DISBALE/ENABLE switch to the right of the audio meters in the Timeline window (the gears)?

Here, when the hardware is disabled, FSP is possible.  No rebooting required.

Yes. I didn't know about that before as I never had need for it. Thanks!

Randomly I decided to just rotated my second monitor, open full screen playback settings, click select monitor on my second monitor, rotate it back to normal, and full screen playback function works again!

Haven't had to deal with this for a while, but apparently it's not working still.  Trying to cut software only, using displayport output of 2011 macbook pro to 2nd monitor for fullscreen.  Only way the fullscreen playback settings panel works is when the second monitor has the menu bar assigned to it in Mac Display Preferences.  Otherwise the "current monitor position" settings become 0,0,0,0.   Can't work that with menus on 2nd monitor.  Any solution?

I'm using latest Mac and Avid software.

Do you have the second monitor set as the main display ???

In what way?  In Sys Prefs, my laptop has the menu bar, and the 2nd monitor doesn't.  That's what causes the problem for me for fullscreen.