With the command Set Material can apply a material to a part or a selection of parts or assembly.
Drag the command on the part to which you want to set the material.
If you want to assign the same material to a selection or an assembly, first select the parts or assembly and then drag the command to an empty area of the scene.
In the window that appears you can select the material to be applied by selecting it from the drop-down menu Material name.
(The list of materials is stored in several files that you find in the folder .. IronPROXT \ Materials)
Also you can set the weight (clicking on the icon which appear) and in this case the density will be calculated accordingly.
If the selection is a assembly, you can set the material for all the parts of the assembly, while clicking on the Kilogram icon will show the current weight.