when you add a plinth height it shows up as a red volume. You need to stay on it ,right click and use add plinth . There you can choose the sides ad recess the plinth should have . Once done they will become visible
I am using 6.04a and when I create a plinth, it isn’t visible unless I mouse over where it should be in the front or left view. Then it shows up as red but still isn’t present in the cabinet or 3d windows.