Answer

 You have been sent the correct part but there is a bit of a knack to fitting it, in order for it to make a watertight seal.

 The Drainage pipe does fit very tightly to the rigid outlet pipe and the end of the drainage pipe needs to be heated first, using a hair dryer, to soften the plastic. It then needs to be turned or 'screwed' on to the rigid drainage outlet. It will fit more easily when turning the tube in one particular direction (i.e. clockwise or anti-clockwise) but won't fit so easily when turning it the other way. It should be fairly obvious which is best when you try it.