When you find summer ( your partner) entie will jump down from a hill at you, battle him to get the emblem. entie can smash rocks in his way
raikus monument is in the cave on rainbow's island the second island you went to.
You can't get a Raikou in Platinum. Sorry. The Legendary Beasts of Johto have to be traded from Heartgold and Soulsilver! These games are hopefully coming soon this Spring or Summer (the English translated one).