I would Need a Long USB (3?) cable, mhhh .
I've found the 5V Input at the cable box is only for the USB port.
With connectet power plug, now a blue LED is lit next to the USB2 port.
And it works!
USB2 is enough?
Why does Leap-Motion have a USB3 connection?
And the plug is very large.
At the recommended place in the center of the glasses, it can obscure IR LEDs.
A small and short USB-2 plug would have been better.
At a DK2 the best place seems to be above the imaginary line of the two triangle corners.
There, no LED is hidden.
The LED is only visible if you look through a camera (here smartphone).