Help with editor--adding Leopards as a Beast Rider Mount
 

I really can't make heads or tails of the editor at all. I'm trying to make Leopard a mount option for Small Beast Riders, similarly to the way Lion is an option for Medium ones.

Leopard is in HL as a companion option for Druids, Rangers and Mammoth Riders. I cannot figure out how to edit the record to add Beast Riders, or copy the record to make a "War Leopard" with identical stats for Beast Riders.

I assume you're talking about the Cavalier archetype, and not the feat of the same name. What you need to do is add the CompList.arCavBeRid tag to the leopard animal companion race (anLeopard).

In the editor, in the general tab group, the final tab is "extend thing", and that will allow you to add that tag to that pick.