It looks like the same model I have used/wrestled with/cursed at many times. Make sure you are opening it to it's full width. The nylon fabric should end up very taut, with no slack or wrinkles at all in the assembled cot.

After fully opening the cot and attaching the first end bar using the most distal holes, there should be enough slack to attach one side of second end bar to one of the side rails of the cot. At this point, try standing the cot on that second end, and stepping on the unattached side of the second end bar. While standing and holding the sides of the upright cot, stretch cot's fabric and work the side rail onto the remaining hole in the second end bar that is being held down by your foot.

Hope that helps.