A couple of different ways to do this: Go to Analysis, Model Analysis, change to Mass Properties and click Compute. If you don't have a material file or simple density assigned to each part it will prompt you for it's density. To specify the density manually you will need to open each part and go to Setup, Density and key it in. Save the a part and repeat. Best practice to use material files which contain a bunch of useful info for more in depth analyses and Mechanica.