To get the Pokemon TCG Pocket Mew card, as mentioned above, you need to complete the Kanto Pokedex, which means getting at least one of every of the original 150 Pokemon in the game. Once you do so, an option will come up in your "missions" tab to complete the quest and reward you with the Mew card itself, like any other mission reward.

How close are you to completing the Pokedex in Pokemon TCG Pocket? Im still a way off, despite owning over 1,100 cards. Its basics like Kakuna and Nidoking that Im still missing, waiting for trading to be implemented because Im reluctant to spend my pack ...

All of the Secret Missions inCG Pocket listed, including the cards you need, and their booster packs, for the Pokémon Pocket Secret Missions.