thank you very much!
it also worked fine using the following command in the menu information/advanced simulation/solid properties check/ and selecting meshes.
as a previous user of NE/Nastran with femap I was used to check the total weight of a mesh by just selecting all the elements.
Each element whether a beam, shell or solid has got material properties assigned (and of course densities) and I thought it was something easy to check for NX to sum up...
