I didn't get at all how to use the Ikanda by the in game pedia. In this page, it says "[units] have access to the special upgrade path when built in a city with an Ikanda". Is it the only way to receive the Ikanda's bonuses, by creating military units within the city with the Ikanda? It also says, "if you manage to acquire all three promotions of the chain, you will get a very nice army!". How do I "manage to acquire" the promotions? Thanks.
Units built in a city with an Ikanda get a special upgrade (Buffalo Horns) when they are produced.
Due to the way upgrades work in Civ V, having the Buffalo Horns upgrade is what lets you later take the Buffalo Chest and Buffalo Loins upgrades.
So to acquire all 3 upgrades, you simply need to train one of your units up to level 3.