The purchase of respondents-credits can be done on your Sharing-Data interface.  If you want to buy respondents credits for the first time, you have to open an account from the Sharing-Data main website.
If you simply want to add respondents credits to you current existing account, it is important that you be already logged-in while you do so and go to the "My account" section. Only the people who have the adminstrator rights on that part can access that feature. If this is not your case, please contact your adminstrator or the person who has those rights.
Click then on "Buy respondents credits" and follow the instructions. The payment can be done directly on line through a secured interface.