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Thanks for getting in touch. I'm sorry to hear you're having trouble.
I checked with Lesa, the author of "Photoshop CS5: The Missing Manual," and she said that the trick is to double-click the Background layer to unlock it. (If you need more info on Background layers, check out pages 84 and 85.) Doing that should make the Puppet Warp command available.
I hope that helps!
Assistant Editor, Missing Manuals