I'm back guys.
I restarted playing it again for I too didnt caught darkrai and cresselia. Got a question for those people who wanted darkrai and cresellia...
Did you open or touch the coffin on the Underworld event?
That coffin itself is the event to start darkrai. So for those people who did'nt touch/open it, means that you have to restart it because you can only go the the underworld once.(Missigno Event)
If you opened/touched it:
Get a munna on Sevii Island 1, Talk to the sleeping boy on Valencia Island, Go to Southern Island, Defeat Cresselia(Cant be caught yet), Go back again to the boy in Valencia, and battle Darkrai(can be caught). After you caught it. Cresselia will be catchable now in Southern Isle. That's all.