PMD2 Explorers of Time Question

[Destiny Hope]

It's probably something really obvious, and this might sound like a dumb question, but how do you get back into Palkia's dungeon after beating him the first time?

 

[Trap-Eds]

Hmmm..I can't remember well, but I think you're supposed to defeat Darkrai first.

 

[Destiny Hope]

That would explain a lot, considering I haven't exactly done that yet...