Since you're probably new to Mega Evolution, I'll give a more serious comment.
Mega Evolution was introduced in the Gen 6 games (Pokemon X, Y, Omega Ruby, Alpha Sapphire), and can also be used in the Gen 7 games (Sun, Moon, Ultra Sun, Ultra Moon, certain Megas in Let's Go Pikachu/Eevee (although I don't know which ones). Black, White, Black 2, and White 2 are Generation 5 games, meaning they were made before Generation 6, meaning there is no way to Mega Evolve a Pokemon in those games. Mega Audino in particular was introduced in Omega Ruby/Alpha Sapphire, so it can't be used in X and Y.
To Mega Evolve, you need a Key Stone (which is usually given to you during the events of the main story (Generation 6) or in the late/post game (Generation 7)), and a specific Mega Stone (for example: Gardevoirite, Venusaurite, Alakazite), which are usually scattered throughout the game. For Audino, you need the Audinite, which is obtained in the Battle Resort in ORAS (Omega Ruby/Alpha Sapphire), was obtainable via an event in Sun and Moon, and is obtainable via the Battle Tree for 64BP in Ultra Sun and Ultra Moon (all post game except for the event, which has ended unless there's some way to get it that I don't know about). So unfortunately, you aren't going to be able to use Mega Audino in a playthrough, unless you have a friend who can trade you a Audino with a Audinite in ORAS so you can Mega Evolve it when you get the Key Stone. (However, I wouldn't recommend using Audino because it's kinda... bad, but if you really want to, that's your choice.)
Sorry for such a bad explanation, but I hope it helps, you can learn more about Mega Evolution in other places like this:
https://bulbapedia.bulbagarden.net/wiki/Mega_Evolution
Oh yeah, if you saw someone using a Mega Audino in Black 2, it was most likely a rom hack, which we at this site don't really deal with.
(Also mods is this question against the rules?)
