Forgot to say.. I'm using FM Pro Adv 12 on Windows 7
You might post the calculated expression that you tried to use.
For a single monitor system, I've used:
( Get ( ScreenWidth ) - $LayoutWidth ) / 2
( Get ( ScreenHeight ) - $LayoutHeight ) / 2
But the variables, $LayoutWidth and $LayoutHeight have to be given values before you use the above two calculations.