It's very straightforward if you wanted to do it rather than print them out yourselves at home.
You buy your tickets on line then you go into any Fnac with your order number/confirmation email, a photo id and the card which you used to purchase the tickets.
They check your details and print out the tickets and give them to you
Yes you can buy and print out at home - that's what most people do - Your printout is used as ticket and it has a barcode on it which is scanned at the park entrance and then again for FP.