spray arm attachment?

The upper spray arm on our hotpoint df61 has fallen off but I can't work out how it is attached to the rest of the machine.

We ordered a new part (506 on their diagram) but it won't click onto the bit at the back of the machine (507) - any ideas ?

A picture of the bits can be found at :

http://www.hotpointservice.co.uk/Service/diagrams/03370-20.pdf

Any help would be much appreciated