There are two ways to do it:
With Macro: Modify the macro here to show the mass property:https://forum.solidworks.com/message/346767#346767
For active assembly Without Macro: Click on Tools > Assembly Visualization. Now in the Assembly Visualization tab, set the parameter as mass. Now Right click on the Mass (on anything there) or click on the arrow and select Save As to export the result/values as excel.
For active assembly with macro: https://forum.solidworks.com/message/432612#comment-432612