The 2nd hard drive slot (which fits into the 5.25" bay) is actually 2 parts. The FRU numbers for the parts are:
45K6369 - This FRU contains the black ventilation bezel and a single 5.25" rail. The 5.25" bay looks to be designed to only need 1 rail with the other side of any 5.25" drive being held in place by friction. Refer to page 25 of the M90 User Guide (page 33 of the pdf) to see how the rail will install onto the HDD adapter.
45K6209 - This is the actual 3.5" to 5.25" HDD adapter. The adapter has a single 40mm fan that needs to be plugged into the motherboard. The location of the only open power connector for the fan is right above the SATA connections. Refer to page 11 of the M90 User Guide (page 19 of the pdf) for a picture of its location. In the diagram, it is marked as item #19. To access it, you'll need to unclip the original HDD mounting bracket and lift it up so your hand can fit.
Overall installation is pretty painless. The only thing that I encountered was after installing the adapter into the bay, closing the front portion was a bit tough since its a very tight fit. You'll want to nudge the cables around so that the case can fully close and lock into place without pinching any wires.