Create a part.
Go and run "Apply Material" command and select PartBody. Select any catagory and chose a material. Click "Apply Material" button to complete.
Go and run "Measure Inertia" command and select PartBody. You will see the mass. In addition if you go part properties you will see same thing.
If you want to change material properties. Double click to material (or right click and select properties) in specification tree. And go Analysis tab and change structural properties.