This option adds nodal masses to structure nodes for structure dynamic analysis or for static calculations when forces are generated for body loads (body forces as well as centrifugal and angular acceleration forces) and self-weight. Open the Nodal Mass dialog by clicking 
in the Load Definition dialog.
You can define the m masses, rotation masses, and those acting along the X, Y, and Z axis of the global coordinate system in this dialog. The value of the mass is determined using its weight (acceleration of gravity is applied here). Also, you can define the angle at which the mass m acts.
If Apply to All Cases is selected, the defined mass is added to all dynamic cases defined for the structure.