Ok guys,
I accidentally solved the problem. It's weired but check it out:

1) Install SE ST7
2) Install Keyshot 6 and activate it
3) Install Keyshot 5 without activation
4) Install the Plugin while SE is running
-->> Tataaa, the Update Button now exists
5) Open a part
6) Klick 'Render' in SE -> Keyshot 6 starts
(6a) If you now klick Update it says "LiveLinking is not running")
7) Start Keyshot 5 (you don't have to klick sth. Just stay in the activation Dialog)

Now you can update your Model in Keyshot 6 by klicking the Update" Botton.
I'm happy now, but who will give me back the last 3 hours
