It looks like the Sky kit uses Socket Head Cap Screws in order to fit the bolt head sub-flush and still have room for the wrench to tighten them. Definitely give them a call to sort it out if you seem to be missing hardware. If you're in a rush and happen to have a good hardware/fastener supplier in the neighborhood, figure out the length you want and the thread pitch is M10x1.25. A SHCS will default to Class 12.9 and will probably only be available in a black oxide finish
In order to reuse the OEM fasteners, you need to take a different approach. I'm in the process of doing just that. Take a look at the attached pictures.