toolbar button or selecting "Self Weight" from the Loads menu and then entering data into the datasheet as explained in Self weight data.Self weight or self mass can be input into any load cases by simply specifying the acceleration due to gravity in any of the three global axis directions.
You can open the self weight datasheet by clicking the toolbar button or selecting "Self Weight" from the Loads menu and then entering data into the datasheet as explained in Self weight data.
See also Datasheet Input.