Missingno is not a real Pokemon, it was a glitch in Pokemon Red and Blue that made a random collection of pixels appear.
Part of the reason it appears is because of the limited space on the Gameboy cartridges (about 16 MB I think it was). So there was a lot of shared data which meant in certain circumstances it became corrupted. This is the same reason whyit's possible to obtain Mew (or any Pokemon) in-game.
The later games were completely rewritten and use different hardware so the above event doesn't occur at all. So there is no Missingno in Black/White, and it's not possible to obtain it, cheating or otherwise.