The way it works for my account (a standard prepaid Mobicarte) is:
Start from login page "espace client" (client area)
Click on "mes options / bons plans" (my options / good plans)
You will see two tabs within that window, one "Gerer mes options" (manage my options) and another called "Souscrire une nouvelle option" (subscribe to a new option). Click on the Subscribe one and a list of all the valid options for your Sim will come up. Click on "Souscrire" against the ones you want to subscribe to.
In my account shown below I am already subscribed to Internet and Mail.
The "Subscribe" tab:
