If you mean Black 2 and White 2 then go to Nimbasa city. Then go upper right through the thingie mawhatsits. then go through the grass into a openinig something or other. then go all the way up should be a hiker I believe and then talk to him and boom you got snarl. there is a tm page on the home page F.Y.I. but if you meant black and white 1 then the other guy answered and you need an event.
Obtain a Lock Capsule from HeartGold / SoulSilver via the Relocator and make Mr. Lock unlock it for you. Snarl will be inside.
As the Lock Capsule was never released, there is no way to actually get Snarl in White.