take it the infra red type of key !
if you know the key fob is working ( you can test this with a radio tuned to long wave) easiest
Then once you have entered the last number via the central locking button
turn the ignition off take out the key and hold down the central locking button constantly and press the key fob on off towards the interior mirror
this will resynch the key
may take a few tries but does work
Renault megane Rt owner, May 2008