Every time I've been in Paris, I've had my own bicycle, so I haven't need to use anything else.

Are you referring to this:
http://en.velib.paris.fr/

If so, it seems they might have made the rental process a bit easier. We had a look at it in 2007. Back then it appeared that users had to buy tickets from a participating shop nearby (so you had to find the shop), and all credit cards were charged $150 plus the cost of the bicycle rental. The $150 was returned sometime later. Now it appears that only certain cards are charged that $150.

I would recommend learning a bit of French, if you don't already know it, to assist with reading the instructions.