If you live outside of France, you have two ways to register for the ride. One is to go over with a company--I don't have any names, you can find them on the Etape site. Certain tour companies apparently get a certain number of slots. (I think Velo Echappe does this.)

My husband found a super-nice French guy on another forum two years ago who offered to sign up Americans. You must be in France to register (with the above exception) because it fills up really fast. You have to mail in your form and money the day that the registration form comes out in Velo Magazine, which happens in January. This year they received over 12,000 requests, and gave out 8500 numbers. All you need, other than Euros, is a doctor's statement saying that you are fit to ride. Plus a friend in France who will forge your signature (did I say that?).