changing the belt of a hotpoint aquarius 1100 WM63?

Help please - trying to simply put my belt back on - which had slipped off - when the machine is on the nut on the back comes off and the belt loosens and comes off again - the nut is slighty damaged - I wondered if that might be a factor and also there is a hole in the middle of the thread bar (sorry dont know what its called) that the nut goes on and wondered if there might be a missing screw that helps to hold the nut on. Any suggestions would be gratefully received.