Captain Cola shows up once enough Cola has been sold in your Zoo, the way to achieve this is to A. lower the price of your Cola.
I lowered mine to .08 and even though it was marked as "Cheap" people were lining up when they got thirsty.
also make sure that B. you have enough of these machines around, i had one probably about every 20 paces from the last, more machines = more profit = more likely chance that Captain Cola shows up