I'm sure that when (if) this plugin eventually works, it'll be all daisies and ponies.
Until then, you'll have to walk through spades of convoluted, incomplete and wrong setup information, provided by Facebook themselves or others.
I'm confident that the "walk-in-the-park" howto worked at one point, but it doesn't correspond with how the world — and the Facebook app creation pages — look now.
To put it simply: It's impossible to follow Facebook's guide, because the places and details it guides you through don't exist anymore.
Add to that that the need to explain what exactly it is you're doing, why, and what the final result will be (apart from making the WordPress plugin work, of course), is zero. So trying to figure out yourself how you should tackle the new strange interface options is hopeless unless you're already a Facebook App developer (I guess). I'm a professional web developer, but apparently that's not enough.
A horrible, horrible experience.