You first need to unlock the Surronded Sea area (a Sunflora mentions about going there and finding treasuere), there you'll find a Wonder Egg that will hatch into Manaphy, when Manaphy will return to you and mention Marine Resort. Then you can go to the Marowak Dojo and enter the Final Maze, Jirachi will be there at B23F.
You can only get the secret slab on Pokemon Mystery Dungeon if you do the following things I tell you to do./Wait for a mission that says,''Help me find the secret slab!''Take it.Then,do the mission and that he/she Pokemon will give you a secret slab. That also helps getting the mystery part. P.S. You can get Celebi on explorers of time, Mew and Jirachi on explorers of sky,Shaymin too. Manaphy and Phione count on every game except for red and blue rescue teams. By the way...How do you get all of the friend areas in red and blue rescue teams?....
you do a misson with umberon
Obtain a mission that allows or has a reward that involves getting a secret slab.
You win the game (post ending). get the secret slab or mystery part (same as time and darkness) go to mystifying forest floor 17 and he might be there if so you have a 14/100 chance of recruiting him
The secret slab enables you to meet legendary Pokemon after you reach the marine resort.
there is a wonder mail code for it
If you get a mission with someone who wants to find it with you, you can get one in the dungeon.
Whatever you want. Recruiting Celebi doesn't do anything to your game, you use her as you would any other Pokemon in the game.
I used the Wondermail Generator online to obtain it.
He will be at Amp Plains, as long as you have the Secret Slab with you.
Secret Slab, or Enigma Part.
Darkria is in mystafying forest so is celibi jirachi is in final maze so is swecune and moltress deoxys is in shimmer hill. Zaptos is in amp plains Enti is in dark crater richu is in consealed ruins