Pokémon Mystery Dungeon: Explorers of Time and Explorers of Darkness/FAQ

Why Can't I Find/Recruit Certain Legendary Pokémon?
In order to find certain legendaries (Darkrai, Latios, etc.), you need to have the Secret Stoneplate or Mystery Part in your item bag. These items are offered as rewards for certain optional missions.

I can't find any Oran Berries! What do I do?
The Marowak Dojo has lots of Poké and Oran Berries. Just go there a few times to get a lot.

Why can't I enter Hidden Land or Temporal Tower?
After beating story mode, you can revisit the Hidden Land by talking to Lapras at the beach. To get to Temporal Tower, you need to pass through the Hidden Land. Very hard, indeed. However, your prize for doing this is a fight with Dialga. If you win, Dialga will join your team.