Board Game Arena

I don't see a way to enable the expansion - am I blind? Probably. Can someone point me in the right direction?

If you're using 'automatic mode' then I believe there is a cog wheel to change settings, otherwise for manual:

It's in the game options. When you start or join a new table all the options are on the left-hand-side.

It also appears on the table listing, although any new table gets filled pretty quick.

In the game settings (at the same place where you select the thinking time available to each player per turn, where you can select if you want to play in training mode or in competitive mode, etc), you can select the Agora expansion.

In Automatic, you need to click on the cog on the left of the game's image to parametrise your settings.
In Manual, you need to be the one administrating the table, and you should be able to change those setting with the panels at the left of the screen.