What are combo aliases for and how do I use them.

You can change the size of the alias, what layers its composed of, if any, and then construct the alias by picking combos for each part of the alias. After making a combo alias, click done. Then when editing a screen, press the O (as in 'oh'😉 key until it says 'Combo Alias' (or 'Alias'😉 on the top right of the screen. Now you can pick an alias, which appears as a thumbnail on the right sidel, and then place it on the screen very easily. Pressing the spacebar will change the orientation of the alias, moving the little 'x' on the thumbnail to a different corner to change how its placed.

Two things: That didn't tell me what they're for, and clicking the screen with one selected doesn't do anything.

You must be clicking a 'blank' one, one that you already didn't create. What they are for is making large 'objects' so you can quickly place multiple combos made at once. You can create an alias, which is essentially a group of combos that you put together by going to 'quest->graphics->combo alliases'. After editing them, you can then put them down on the screen. When you have one selected (when in combo alias-editing mode), it will be previewed on the screen as you move your mouse around the screen you are editing.

Ah I finally get it now and my problem was I was trying to draw on a non-existant layer.

Yeah! You can use combo aliases for placing furniture, pools, and columns etc. Pretty COOL huh!