If your absolutely sure it went in the machine and its not in the drum-it will probably have found its way down to the sump filter mounted in the big rubber hose under the drum adjacent to the pump.
Unplug and pull the machine out. You may have to turn the water off and disconect the hoses. Remove the panel at the back to get at the hose leading down to the pump from the underside of the drum. It'l be in there somewhere.
Good luck!
GB in Wales, March 2013
