To adjust height of walls, you have to be in 3D aerial view, you can only change single wall (divider) height and room's height.
Select either a single wall (divider) or a Room, then select, in the contextual menu, the option "Height".
You can use the slide button or enter the value you want.
The value has to be between 0,30m and 5m (1' and 16' 5'').
It is not possible to change the height of a single wall of a same Room, the cursor will not appear, all the walls of a Room are at the same height.