I have an Eclipse 12 gal. so let me take a stab at it. The only think I can think of is the little rubber "lip" that forms the leading edge of the filter media tray. The pump has a "notch" on its bottom side that rests on top of the "lip" of the tray in order to secure the pump/power head from migrating off its attachment. I can't rotate the symbols but it should look something like this:
> lip > notch (notch rests on lip to secure pump) To make a little more sense, rotate the symbols so the point is pointing in an up direction.
Geez, I'm glad I'm not in charge of writing directions for anything. I hope I haven't completely confused you. If you're still unsure of what to do, Marineland has excellent CS and I'd recommend calling their 800 number for help.
Good luck.