You'd insert it into both show_on_uncolonized and potential. But that's only if you want to restrict normal empires from using it, which isn't strictly necessary. The UI has space for 5 district types, but in vanilla there are only 4 types available on an Ecumenopolis, so I set it up so the Commercial Arcology would be available to all non-gestalts.
The only reason to restrict it would be if you really want it to only be available to megacorps (though then you might also want to make it available to Merchant Guild empires with an OR clause) or if you already have another mod adding a different 5th district type. But in that case you'd need to make one of the other arcologies unavailable for megacorps by isnerting is_megacorp = no in the conditions or use a NOR clause if you want to do Merchant Guilds too).
That's actually exactly right, I just want to make it available to Megacorps and more specifically - only to me
I managed to insert it into the game, but there's a description problem. Also, i reformed out of a Megacorp and into a normal empire, but I modded it before I saw your reply, so I'll correct that one.
Not sure about the description issue though.
Also, I see there's no adding jobs in the description (only after I reformed into a normal empire, it gave jobs normally when I was a Megacorp), so I guess the place I added "is megacorp = yes" affects whether it gives jobs or not, but not the existence of the arcology. Interesting.