It's definitely not there. There's even a key next to it on the PSU itself to show you where the cable should be, and it's just not there. All of the cables on the back of the PSU are detachable...
I also seem to be missing a PCI-E cable, but that isn't really needed anyway since I don't have that many expansions.
I've just emailed the people I got it from. Hopefully they'll be able to just send me a cable for it, but I'm not gonna get my hopes up.
@ Astro, I just checked the manual and the manual seems to indicate that it should be there, as well as the relevant technical specs on the site I got it from, but it's not there. I haven't tried attempting to boot it, though. Do you think it's worth bothering?