OK there is a knack to put them together so this I found works best:
Put the feed tray into the left hand side shell (as you look at it from the back) and click into place.
Put the lid into the left hand side shell.
Put the right hand shell on making sure you start by putting the drive cone pin in first then squeezing together from the front to the rear of the hopper clicking into place as you go.
It is a bit of an art but very very easy once you get used to it