To access this mode, you have to finish Adventure Mode twice, and then a button labeled "Forever" will appear on the Main Menu screen. Once you have clicked it, it will be a new, playable mode.
During the boss levels in Pokemon Trozei!, the bosses drop annoying little capsules that you can't trozei, even if you line them up with other white capsules. I found a way to trozei them though! If you line them up with a ditto, you can make them disappear!