Is this the same for a 2005 Scenic? And by black button do you mean the one by the clock?
Press the button on the end of the right hand stalk to scroll through the trip computer screen.
You will come across the one that shows a spanner in the bottom left corner, this is the "time to service".
When you have selected the "time to service" screen, press and HOLD the black button on the intrument panel.. It will start to flash after a few seconds, keep the button pressed until 18000 appears and flashes..
Then release the button before the 18000 stops flashing.
The screen will now show 18000. (18000miles till next service), Job Done..