Under Pokemon Challenges on the main menu, scroll down to the bottom, choose one of the green custom challenge entries, and delete one so you can input the keyword "PHSKUTDF" (without the quotes). Doing so will allow you to unlock Genesect after you finish the 20 question challenge. You don't have to get any of the questions right; you simply need to finish the challenge. Afterwards, Genesect will be available for you to view inside Pokedex 3D Pro. Its signature move, Techno Blast, will be unlocked as well.
Not only is the Pokemon Genesect available in Pokedex 3D Pro, but if you happen to have Pokedex for iOS, you can unlock Genesect on that as well! To unlock Genesect on Pokedex for iOS, all you need to do is search for "SNMPSCTP" in the top-right corner and you will unlock Genesect.