(And the above website has loads of information on what else to do in Paris, with some information available in the English version of the website as well).
And here's how to signon :
Ok, if you needed help with that... here's the Google Translate output on the useful part:
1. Identify signage Paris Wi-Fi with Wi-Fi logo Paris
This indicates that you are in the area (of) an oasis Paris Wi-Fi
2. Turn on your Wi-Fi equipment
3. Select the Orange network.
4. Start your regular internet browser and type any website address (eg http://www.paris.fr).
5. Select your pass Paris wi-fi 2h offered by the City of Paris and the Ile de France
6. Now you can surf freely on the Internet, send and receive emails, and all this without needing to be connected in any way!