The Inyangamuagyo is a Courthouse replacement; normally they're impossible to build in your own cities, so it's a lot more production time spent midgame to get the most out of it unless you take a very specific (and in its own way suboptimal) build order. As for the Abiiru, the bonuses are additive, not multiplicative, so if you luck out and get five Science boosts in a row, it, er, gives you the equivalent of a National College or Observatory. At considerably more cost. And it's dependent on you getting a Great Prophet, which is by no means a sure thing on Deity.

However, if you want to get the mods and do a bit of testing (there are a couple of bugs that people are finding, I can't fix them because I don't code), there are links to the direct downloads from Dropbox for each Civ on its Steam Workshop page. Those should work on a Mac, but I wouldn't know, I haven't used one since I was in primary school. =]

You have to get someone else to code up the individual stuff, but that's generally speaking the easy part. Each Civ has a unique attribute (a passive bonus) and two unique things it can make in game, be they units, buildings, or tile improvements. The Dutch, for instance, can build Polders on Marsh or Flood Plains tiles, and have a Caravel replacement (an early mid-game melee naval unit) called a Sea Beggar. This can be tricky, but you still have a lot of other things to do before you can in good conscience release your modded Civ.

You then have to knock together art assets, especially if you have unique units. This involves an icon for the Civ in question, icons for the unique units, portraits for all the uniques, a map for the title screen that shows the relevant borders, and a leader screen. Civ 5 has dynamic, 3D, animated leader screens. There are maybe, like, two mods that have those, and only one of them has independent voice acting. By total coinkydink, that's one I also worked on in a minor capacity: Australia, by the Colonialist Legacies team. You also need to do the Civilopedia entries (background information on the Civ in question, its leader, and the unique units, buildings, &c), unique leader dialogue, and a Dawn of Man speech. This last part is my particular speciality; a bombastic speech meant to inspire you to victory as your chosen Civ. I love writing them, they're so pompous, it's wonderful. =]

Leader traits kind of went out the window in Civ 5, but there's a mod that brings them back, because of course there is. However, you do have to code particular AI flavours for when you're playing against the Civ - how much they'd use nuclear weapons, how loyal they are, that kind of thing.

Actually, that's a point - there are a LOT of mods for Civ 5 now, and compatibility with them and support for them is greatly encouraged. Events And Decisions, Cultural Diversity, Ethnic Units, and the aforementioned Civ 4 Leader Traits mods are the big ones, but there's also Exploration: Continued: Expanded, Rise To Power, Historical Religions... let's just say there's a lot to keep track of. There's also Mercenaries, which has just been released and is pretty awesome, but none of the Civs in my pack support it yet.

After all that, which is quite a bit of work, you're ready to release it onto Steam or direct download via Dropbox and have people report a shedload of bugs you missed and (if Colonialist Legacies' experience with our Afghanistan Civ is anything to go by) occasionally draw out all the crazy, racist rednecks who call you a terrorist sympathiser for having the gall to depict Afghans as something other than skerreh brown peepul to be Gouda-cheesed out of existence in interchangeable modern military shooters.

But hey, it also means you can nuke Thatcher, so Civ 5 mods rule. =]