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Thanks for your question. I believe the problem is that Snapseed is not one of the apps that has been updated to work with the new Extensibility features of iOS (note that the book says "if you don't have any such apps, the ... button doesn't appear).
Because the book was published before every photo-editing app would have a chance to update, a complete list of apps that work with this feature didn't make sense at the time of publication (and would be terribly hard to keep updated anyway), but the two examples explicitly mentioned in the book (Fragment and Camera Plus) were tested and confirmed to work.
I don't know which other photo-editing apps you're using, but I have a feeling they have not been updated to support this feature either. If you're looking for a free app that's known to work to test the feature, you might try Litely. Like you, I myself could not get Snapseed to show up at all in the Edit menu, but Litely comes up just as Camera Plus and Scanner Pro are shown to do in the figure on page 235.
Perhaps Snapseed will update to support this feature in the future, along with the other photo-editing apps you use. In the meantime, I hope you find this explanation of its limitations helpful.
Senior Editor, O'Reilly Media