OK,  I had the same problem and it drove me up the wall!  Here's what needs to be done.  Install the little plank part that slides down into the grooves on the tender.  Then try to install the oil bunker again.  It should go right in.  Don't ask me why, I guess it just lines things up right in a very tight space.  Let me know how it turns out!

Cheers,

RJW