Smallest sprocket doesn't fit on freehub?
It doesn't really matter if your freehub body is made for 10 speeds, since Campagnolo has the same body for cassettes between 9 and 12 speeds. The body does not go through the smallest sprocket.
The splines on freehub body are not identical and there is one position where the cog fits properly. The lock ring should be not tightened if the cog is not in correct position. The easiest way to find the position is to rotate the loosely sitting cog. Once in correct position, it clicks into place.