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 -0.030 ksf (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. (Don't get irritated by the two surface loads already shown in the workspace as LC1 is still active there.)
Click Apply & Next again.