Well, as you said, the Secret Slab is an item which lets you encounter and possibly recruit certain Pokemon, like Celebi, Lugia and even Darkrai. The only thing you need is the Secret rank, and you will also need to know what dungeon to look in.
The easiest way to get it is through a wonder mail:
@WW0 568@ 7%1R
9=-N 1&M4 9&-1