You can literally type the values in the fields. I had been using 1 [for density] and 1 [for volume] for all objects regardless of the mass I had wanted to "roll-up" to next levels. The "mass field" values was being ignored and all objects were rolling up as being 1 lbf*s^2/in!!!.
Now I leave density at 1, and make sure that the value entered in the volume field is the same one entered in the mass field. That will make everything work just fine.
Incidentally, most "next assemblies" have some subcomponents that are "fully assigned" and others that are "geometry and density" and still others set to "geometry and parameters". For all that to get summed properly in these next assemblies (using the work around described in this discussion) you must be sure that the property setting for these next assemblies is set to "geometry and parameters".
