So you want a one-button macro that will summon a flying mount where you can, and a ground mount where you can't?

WoW changed the way that mounts work...they're stored on a Pets tab on your Character screen now, not in your inventory.  So rather than the 'use item' macros of old, you have to call the companion based on its position in your pet list.

Try this:

/run if IsMounted() then Dismount() return else if IsFlyableArea() then CallCompanion("MOUNT", <flier pos>) return else CallCompanion("MOUNT",<ground pos>) end end

<flier pos> is the position in your Pet list of your flying mount, and <ground pos> is the position of your riding mount.

Don't use the <> symbols.  The command would look like ("MOUNT", 1) if your flying mount was in the first position in your pet list.

So enjoy your one-button macro to summon either a flying or ground mount!