Instructions
Get the Pokedex from Professor Oak while he's inside of Professor Rowan's lab. The Pokedex allows you to register all of the Pokemon you see during the game. Wander around Sinnoh until you have seen all 150 Pokemon.
Obtain the PokeRadar from Professor Rowan in Sandgem town. He will only give you the PokeRadar after you have seen 150 Sinnoh Pokemon in the Pokedex. The PokeRadar will allow you to identify non-Sinnoh Pokemon that you couldn't previously find.
Visit route 228 and enable your PokeRada by setting it to a button shortcut. Travel through the grass until you find Trapinch. Be sure to investigate any sections of grass that are moving or flickering with light.
Choose a Pokemon to capture Trapinch after you have found him. The water and grass Pokemon have the best chance of capturing Trapinch because he is a ground Pokemon. Capture Trapinch with a Pokeball after your Pokemon has successfully battled him twice.