I had different M/C but the principle is the same, remove the back cover of the M/C about half way up the drum you will see a plastic bottle, usually white, about 2" long, with small bore pipe to each end. Take it off and clean it out.
easy as that, the water level sensor uses this, if it or the pipe gets blocked with soap (this will probably only happen once in the lifetime of the M/C) the sensor does not know how much water is in the M/C and will not proceed until it does.
Andrew Wootton
October 2010