Did you change the oil and filter yourself? I believe that the process is fairly complex and may even require a tool.
If you had this service done at a shop/dealership, take it back to them and give them a hard time for not finishing the job!!
On 08,09,10 C300 using the scroll arrows on the steering wheel, go over to trip and at same time hold down the "phone button" for 1 second and press "ok".
Now you are holding both buttons, the "phone and ok"
this brings you into the shop menu and then you can scroll down to the assist plus option and hit full "full service" okay,
its at this point the next prompt will ask you to confirm if full service was done, press down arrow to yes, and hit okay, it will then reset.
Hope that helps.