@donform - Welcome!
First, does anyone know of a youtube video or website that goes into more detail about the scenario editor than the game manual does?
I don't off hand but some probably exist, most the ones I'd point you too revolve around lua scripting tutorial of which there are some great one. But there are a host of resources in the mods and scenarios sections of the forums.
with the scenario editor I cannot figure out how to add an airbase.
I think this was in the manual somewhere but... there are two basic ways.
1 - You build your own. You'll need a runway, at least 1 runway-access-point, and at least one spot to store something (hanger, open parking spot, revetment whatever.. Technically a tower and a fuel store and at least 1 ammo storage is optional, but it's wise to include the fuel like the manual at some point suggests. When you have the those units in place and want to 'make an airbase' out of it, all you have to do is select all the units and Group them. To switch between "Group view" and "Unit view", press V.
2 - Just insert a "single unit" airbase (select facility, start typing 'single' and it should bring up ones of various size, this is a self contained unit, attacking or having specific units of this airbase is not possible. IE option 1 lets every thing included in the airbase be targeted by the enemy, this option (2), generally does not allow that, the unit can only be attacked as a whole, and even then only by certain munitions or ai missions who will just randomly target sub-units of the 'single unit'
Custom airbase in unit view...
* The difference is AU\Unit count option 2 technically is just 1 unit, while the usual option 1 take up units that all have to be checked every second. Basically a way to either a) save time if the target isn't meant to be hit, and b) save cpu for huge scenarios in reducing some overhead.
but I cannot figure out how to position the planes on the ground at the particular airbase loaded with weapons & fuel.
? Do you mean put a plane in a specific parking spot, or just do the usual Aircraft Loadout setup? The first, you can't, well actually you can initially but that gets into using option 1 above and individually adding aircraft to specific hangers..etc. The later is as simple as access the loadout screen just as you would for the airbase ingame, so I'm confused by the question.
Adding aircraft to the new airbase, accessing configuration screen for new aircraft

Stuff is grayed out because you didn't add munitions, but can still force it.
Mags and Munitions entries...
Haha figures Whicker beat me to it while I drew up pics.