Hi Jörg,
Update Download and Instructions
After downloading clean retina pro theme from your login account then it must be in zip format. Then go to dashboard-> appearance-> themes-> add new -> upload theme.
Then choose your file (zip clean retina pro) from your dashboard and install it and activate it. Then you can see the clean retina pro file from your dashboard-> appearance-> themes (your clean retina pro file is installed there)
Now you can use more pro features.
Thank you!