In order to help Robinfur, you must become a medicine cat (Get him the poppyseeds and he will end up talking to you at the med's nest). After you talk to him, go to the cold stone and the guards wont let you in until you are an experienced med (You can just kill them tho), receive a prophecy, tell Robinfur about it and then go to sleep. You enter a nightmare and here's where you need to go (click on the shadows til they disapear):
Leader's den
Plumclaw's Patrol area
Continue to the left