You can do it in two ways.
Display --> Show Tables -> Material List. This gives you various options.
If you want to find the weight of the structure for seismic purposes, then you can do one of two things.
Based on how your mass source is defined (as in what loads contribute to mass), you can show the mass and then multiply by "g" (acc. due to gravity) to get W.
Display --> Show tables --> Mass Data --> Diaphragm mass data or Assembled point mass.
HTH, if you have more specific questions, feel free to ask