The OP should be using
certificat-air.gouv.fr
which self translates into English. You will also need a scan (if you have a scanner) of the front cover and inside front page of your V5C document.
You will get a reply almost by return with a temporary Certificate - the official sticker should arrive in less than a week.
Incidentally, the instructions for fitting the sticker are a bit confusing. Just interpret it as that it can be seen/read by an official standing in front of the middle of you car, and to be compliant in the UK (in terms of stickers) it must be fitted in an area of the windscreen that is not wiped - a.k.a. an unswept area. Before fitting, wipe the desired area with meths to remove any grease - or use Windowlene cream to the same effect. Then warm the certificate with a hair dryer or heat gun, and the same to the inside of the glass where you are fitting it. Doesn't need to be hot, just tepid, but when warm, once in place it will stay there. It is related to the vehicle so it stays in place when you sell the vehicle.