Got it, despite not showing up in the Steam list of DLC. Had to go into options menu and enable a couple of the DLCs, including the new one. So don't forget to check there when you load up the game.
The new scenario is absent, however.
Same here as of last night, but I got it figured out this morning.
As I posted earlier, I bought it and then noticed that it was still listed as 'Windows only' in Steam and not listed on the Mac App Store at all.
Then went into Additional Content and found the Indonesian/Khmer civs, enabled them, found several other Mods/DLCs also disabled.
Now this is weird: some of those were Mods/DLCs I was using in a current game, and the game loaded up fine and played even though, according to 'Additional Content' some of the components were Disabled!
WTH?
This morning, the DLC is listed in my Steam library , but as 'Not Installed', even though the Civs are available to play. Clicked on the DLC to 'Install' it, and it downloaded a single file in about 8 seconds, which means, whatever it was, it was nothing like the entire DLC.
Then went into the game and found both the Civs and the Scenario listed in Additional Content, where the Scenario had never appeared before.
So, for whatever arcane reason, this particular DLC requires you to manually Enable all the content at first, and 'confirm' the Installation to make sure you've got all of it. Specifically, the scenario doesn't seem to show up at all until you click and 'confirm' the installation, because both civs were already available, as far as I could tell from a few turns of experimental play last night...