To define the live load of the lower slab, set LC2 in the 'Load Case' area first (1). Use the list again.
Then enter the value of 1.5 kN/m² (see the Chapter System and Loads) (2). Finally, click the button (3) to select the lower slab (no. 4) so that its number is displayed in the 'Assigned to Surfaces No.' area.
Click Apply & Next again.