Autodesk Inventor 2011 help

[Basel A]

First, you may find it easier to find your density in g/cm^3 (or lb/in^3, whatever you're working in).

Second, the easiest way I've found of doing this, though jank, is to edit the mass until the density is adjusted to where you want it to be. It sounds like you're in the right menu, just edit the mass instead of the density, because density is probably in a gray box.