It should have fallen down to touch the access cover plate by gravity. It is very smooth and slippery, so it is hard to get it to slide back up. I usually wipe it with something like brake cleaner so I can get a grip in it. The shaft at It's most inner position will not fully enter the block surface. You can put a socket on the bolt on the front end of the crankshaft and rotate the engine, if you think it is at it's farthest out position. You will have to overcome the high diesel compression so don't go too fast. It just makes it harder. If you listen you may even hear the compressed air leak past some valves. Just remember that when you are tightening the pump bolts, you should be tightening against the pump spring pressure. If the bolts get firm before the pump body is against the block, you may have slipped of to the side of the rod.
George D. Paxinos