The first steps for getting access to the API are :
Upon subscription, an email is sent for confirmation. Once confirmed, just log in.
Get the information of your contract (key hashcode and piece of code) in "My Account".
Copy/Paste the piece of code in a new file named (for instance) "maPageApi.html".
An HTTP Server is needed to serve your first page correctly.
An URL such as http://localhost/maPageApi.html is needed to pass through Geoportal securisation layer and access Resources. The development keys delivered by the api.ign.fr site are only granted to serve pages under the "localhost" domain name.
Once copied in the HTTP server file hierarchy, your page can be seen at http://localhost/path/to/maPageApi.html URL.