Like i said: gameshark and/or gamegenie. I did have both.

Copy and pasted from gamewinners under Game Genie Codes section:

"Find Mew in red version:
151-91A-7FC
151-93A-F7E
151-9BA-5DB
Mew will be in the field west of Cerulean City."

You enter that in the device... then it will manipulate the game code.