First tell Bramblestar about Crystalblaze, then talk to her, then wait a couple of days before Crystalblaze's mate comes to ask you to visit her, then go to the fair. then talk with the can next to the tent, then ask cookie where your sister is, then go into the tent, then you can talk to crystalblaze, talk to her a lot until she offers you to talk to the cat next to cookie if you could join the fair (WARNING: You can never return to Brambleclan).
Or if you don't want to share Crystalblaze's secret then jump under the guard who is guarding the enterence to the fair