Sun,
For the steriliser, just use citric acid or the juice from a lemon to descale the heating plate. Squeeze one lemon then top up the amount to 120ml, pour in and turn on the steriliser like going to sterilise bottles. After the steriliser has cooled down, use a normal kitchen sponge to scrub the heating plate lightly, the scale will come off. Then just wash as per normal.
For the tray, just scrub clean. Unless it is really very badly stained. Cos even if you buy a brand new one, you may end up with the same problem with prolonged usage. I had to descale mine quite often last time cos of extensive usage... about 8 times in 24 hours for the first 2 months cos of breastpump sterilisation!