You do know that you have two bolts to tighten - right? With the cap off, you use a 5 or 6mm hex key and tighten the interior bolt - which expands against the sides of the steerer tube. Then insert the cap screw through the cap and into the interior bolt to pre-load bearings.

It fooled me too - and I had to call Enve. Once I figured it out - it actually seems like a nice design.