good question. i have the beta installed and couldnt find it either. called apple and basically their responses were "what are you talking about?" and "we dont know"
First of all apple is not going to give you any support on ios8 period as long as it's in beta..
You know what third-party means? It means that it is something developed by someone other than Apple. They announced the feature less than five hours ago.. Even if they allowed third party keyboards on the app store before iOS 8 is officials released.. which they won't.. Do you really expect a developer to design, program, beta-test, submit to the app store, and have it approved in under five hours?? Get real!!!