The pic above is a blown up shot of the plugs, the actual lead\connections are very thin and a lot smaller compared, length is the only variation apart from some have 90 degree plugs, but you will need such a lead to check the drive out, power will hopefully be there already, but that depends on your current PSU and set up.
Now was this a store bought system ? if so if you could give the make\model etc, possibly service or tag numbers if present, some makers use these, but not all.
Everest not finding everything is a pain, but not the end of the world, as long as your comfortable shut the system down, remove the mains\power lead from it, lay it on it's side, make sure the window side (if it has a window) is facing up, if it has no window then looking straight onto it, lay it to the right hand side, undo the screws on the rear lip of that side and remove the side panel, these usually slide to the rear of the case before being able to be lifted clear, now the inside should be exposed, make sure you touch nothing to begin, just look.
Now look at the board, somewhere on it the make\model should be printed, make sure you include any revision numbers as well, that should give me your board, now while your looking try and see if you have any of the leads in the pic above, they will be thin red leads, usually anyway, what I am hoping is you will have 2 of them, if so we can borrow one from another connected device for testing the suspect drive.
There is an alternate option available if you are not comfortable going into the system, also if you don't have another Sata lead free this will take care of all that in a risk free method that you may be happier with, and it's simply a USB Sata connector, great tool, allows connection of all drive types via USB which saves going inside and upsetting components, they don't like being moved sometimes, see what you think.
This is the sort of thing I mention, I don't suggest getting this specific one, nor from this seller, although theres nothing wrong with either, but search for your best deal from a reliable source, hope this helps.