Both ES Explorer and Retroarch (as well as others) will install perfectly via the play store. Just find your game/app on the play store, click install, and choose your AndroidTV device. Have only tested with the ADT1.
They will show up under chainfire sideload launcher http://www.androidpolice.com/2014/10/23/chainfires-sideload-launcher-lets-android-tv-users-easily-start-sideloaded-apps/
How do you map the buttons on the ADT-1 controller to RetroArch? There isn't any button mapped to start/select.