Does anyone know how to use ATV with a keynote presentation?

AppleTV was created to work with media : music, video, photos. Keynote is not on the Apple tv.

Create your presentations in keynote and export them as a movie.

Yes use Quicktime. Try to create a presentation that uses a widescreen aspect 1280 x 720 would be good, then export it at full quality which should give you a movie of 720p at 24 fps and should be just fine for the tv, if you have any problems with exporting it this way, you could try manual settings or simply run the export you made at full quality through itunes using advanced/create an Apple tv version.