It's held in by internal pressure and a very tight seal. The waste pipe lip also holds the lid down.
Pull evenly each of the tabs at each side so that they are free - possibly gently ease them with a screw driver. Loosen the waste pipe lip as you forcefully pull at each edge.
David, London
October 2008
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix waste disposal units.