The additive is in a tank which forms part of the fuel tank. it can be filled by raising the off side rear passenger seat. It would normally only need filling at a high mileage when the car is serviced. Perhaps this was not checked at the last service or like my old C5 it did not need refilling, reset the computer by disconnecting the battery, wait 5 minutes, reconnect battery, wait 5 minutes then start car. you shold find the message has gone.