I'm trying to export the center of gravity calculated from the View --> mass objects
How do I export these objects ?

Currently, iPi Mocap Studio cannot export the gravity center animation. It can be relatively easy computed given the exported skeleton animation and bones' masses.
Though, recently we've got the same request a couple of times. So we may add this function to our software in future releases. But no promises until major features are released :)

oh cool :) . So there is a solution. Would you mind explaining how to do this / provide links to resources ?
(Currently using blender3d)

The center of mass of some object group (for example, skeleton consisting of bones) is calculated using formula:
center_position = (position1 * mass1 + position2 * mass2 + ... + positionN * massN) / (mass1 + mass2 + ... + massN)

Blender supports scripting in Python, so it should be not too complex (for a person having appropriate skills :) to write a script doing this calculations.
I have googled a couple of examples for you:
http://books.google.ru/books?id=13fXlLfEM54C&pg=PT94&source=gbs_toc_r&cad=4#v=onepage&q&f=false
http://blenderartists.org/forum/archive/index.php/t-202716.html (links to script seem not working now, but you may try to contact with its author)