Did you ever get it to close? I'm having the same problem! It seems like everything is lined up ok, it just won't close.
" The cover slides just slightly to the left and right (very slightly almost not noticable) and occassionally aligns with the ridges on the inside causing it to buckle and thus not close. Try and offset the ridges just slightly and then close. "
This worked for me!!! Thanks!
The solution is to lay the printer on its front completely, then the back cover will close with no problems. This causes whatever was blocking the way to get out of the way and close. This works, I just did it on two Dell3100CN printers. I had to open the back cover to check for a paper jam and I couldn't get it to close. I opened the reg cover on another dell3100cn printer to figure out how, but then I couldn't get that to close too.