convert mv into g

Go to the data sheet and find out how many mv/g it is calibrated for.

First you go to our host's site and look them up

http://www.globalspec.com/SpecSearch/PartSpecs?partId={DA468D9D-5CCF-49DB-80C4-6FE9364CC970}&vid=175559&comp=5

Then you go to the supplier site conveniently listed there and look at their instructions for for how the signal is produced:

http://www.colibrys.com/e/page/187/

http://www.colibrys.com/files/pdf/products/PD%20MS9000D%2030D.MS9X.C.12.08.pdf

And then you establish the things we can't know, like the input voltage

And with these details in hand, we will help you work the details.