in pmd2:
-zapdos
-mewtwo
-entei
-raikou
-celebi
-latios
-latias
-deoxys
-lugia
-ho-oh
-darkrai
in pmd3:
-zapdos
-deoxys
-darkrai
-latios
-latias
-deoxys (not the one in temporal spire)
-celebi
The secret slab enables you to meet legendary Pokemon after you reach the marine resort.
look it up on google ("pokemon explorers of darkness secret slab") and enter it on the main menu
the secret slab lets you find certain legendary Pokemon in cetain floors of a dungeon. for example, with a secret slab you can find darkrai on floor 3 of the miracle sea.
sometimes on a mission, it will say to explore with the client to find a secret slab, so when you complete the job you get to keep the secret slab or you can just put in wondermail codes to obtain it.
Yes it can
It helps you get all of the legendary pokemon.
You can't, but you can use it in the Final Maze and in other areas you may find legendary pokemon. I've heard rumors thats articuno, moltres, and zapdos live in the Final Maze and you can only get to them with the secret slab in your inventory. Other pokemon I don't know so you can google something like: "What Pokemon can you get with the Secret Slab"
pickle
you must be getting it messed up with Pokemon ruby version. there is no secret slab in Pokemon mystery dungeon. at least in red or blue.
the marina resort
You must have a Secret Slab/Mystery Part. He is in Dark Crater, 10F.
you don't activate the secret slab the purpose of the secret slab is to summon legendaries in certain dungeons