I'm trying to recruit Zapdos in Pokémon Mystery Dungeon: Explorers of Time. He is able to be fought on the 7th floor of Amp Plains if I have either a Secret Slab or an Enigma Part in my bag. I got to the 7th floor and walked everywhere, but no Zapdos. Then I selected 'Use' on the Secret Slab, but it just disappeared and nothing happened! How am I supposed to use it to encounter Legendaries?
Hmm. The only solutions I can think of is either a) you've already recruited Zapdos and just forgot about it, or b) you haven't recruited Manaphy, or c) he hasn't talked to you about the Marine Resort. (But I don't think Secret Slabs appear until you recruit him, anyway.)
Im sorry since this is years after but you don't use the secret slab. The Pokemon appears in the dungeon so when you hit the floor I suggest you look at the recruitment search menu. And I also suggest not using that slab as a consumable.