Instructions
Finish the special episode level Bidoof's Wish to unlock the Star Cave for future missions.
Rescue Scizor in the Crevice Cave mission after you have completed the main storyline of the game. This earns you the Secret Rank that allows your Pokemon team to take on secret special missions that involve legendary Pokemon.
Return to Spinda's Cafe to obtain Jirachi's Challenge Letter, which allows your Pokemon team to return to the Star Cave.
Trek through 15 descending levels of the Star Cave while battling wild Pokemon. None of the Pokemon you encounter here can be added to your team. The Pokemon living in the Star Cave include Gulpin, Lunatone and Poliwag.
Talk to Jirachi on the sixteenth basement level of the Star Cave, and it will join your Pokemon team.