How do you recruit Smeargle in Pokémon Mystery Dungeon?

To recruit Smeargle in Pokémon MD, beat Rayquaza and get the Sky Blue Plains Friend Area. Then, go to Pelipper's Post Office, check the Bulletin Board, and find a mission for rescuing Smeargle in Howling Forest. After you finish that mission, you will unlock Howling Forest, and he will join your team.

In Western Cave, Smeargle can be found on 4-6F at Lv25 with a 6.8% recruitment rate.