It will be in Special Events in the future. Hopefully soon.
An event
You cannot catch Genesect on Pokemon Black or White. You will need to go to a Pokemon Event. No Genesect events are announced yet.
you need wi-fi but the events over
it's not an offical pokemon so you have to use an action replay
genesect can't evolve
Genesect is a legendary Pokémon which means no NPC trainers will be holding one.
In order to get Genesect in Pokémon Black 2 and White 2, trading is required.
Genesect is an event pokemon. You will not find him anywhere in the game.
Genesect is an Event pokemon. You have to get it via Mystery Gift
Wait for a wifi event
You can't without hacking.
Genesect is not out yet. Pokemon.com says that it will come out later as a Wifi event pokemon.