Aha! I turned on experimental updates in the options for the copy of 0.11.6 I downloaded, and it offered me the scenario pack when I went to check for updates. Now the mod is downloaded. Thanks for the explanation!

I definitely downloaded the pack from the website a couple of weeks ago (I thought from the section of the "profile" page that says Scenario Pack but I'm not sure) but now I can't find it either. Based on this post, I also tried downloading the experimental release (0.11.6, Windows 64-bit zip ...