You only need to carry it with you in your treasure bag. As long as the other prerequisite is met (you can access Marine Resort) then it makes certain legendary pokemon appear as wandering monsters. It is different depending on the version you are playing though.
pickle
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.
Recruit from Dark Crater or Mystery Part or Secret Slab
They help you recruit all the legendaries.
It helps you get all of the legendary pokemon.
http://apointlessplace.net/wms/wmgenerate.html create a code to get it i did.
yes on the 10 floor of the spacail rift with secret slab
yes,b18f
latias is in Sky Fissure 10F and Joy Cape B10F but you must have Secret Slab or Enigma Part in your treasure bag
look it up on google ("pokemon explorers of darkness secret slab") and enter it on the main menu
It´s a Heatran (Lv. 49).The Dungeon has 20 floors and Moltres is also to be found there (only in Explorers of Sky) but only if you have Secret Slab with you.Fond this in Pokewiki:PKF&+X-H+M2SHW65J9W0=TTC0PKH+0YPY+Should be for the Secret Slab. :P
Obtain a mission that allows or has a reward that involves getting a secret slab.