From a quick perusal of this I see two key things you need to understand:
(1) Where is the moment of inertia being calculated around? An axis? A particular coordinate system? This will be the fun mathematical part of the endeavor. Hopefully a saved analysis will give you all the model properties you need to do the calculations.
(2) How to save an analysis and subsequently access its values in equations. There are a lot of discussions of this out there, too. I do this on a somewhat infrequent basis, and usually just search for it when I'm unsure of the syntax and steps involved. Like a lot of Creo stuff, it's not particularly intuitive, but then again, provides amazing functionality once implemented.
I don't recall a mass properties analysis making any parameters (measurements do, but polar moment of inertia is not one of them,) but it does create a .inf file, which is plain text. MathCAD should be able to extract a value from that text file.
An alternative is to create a mapkey that runs a script to generate the .inf file, runs an external program to extract the value from the .inf file, and then generate a further mapkey that sets the value of a parameter equal to the inertia, and finally runs that mapkey to create/update the parameter.
It's also possible that one could use a program like AutoIt to monitor the output area for .inf files and, when one shows up that includes inertia values, it creates a mapkey to do the assignment, allowing the user the choice of when to run it.
Perhaps there's a more direct route, but I don't know of it.