I have solved the build problem. There is a step missing in the Screen-Cast: Introduction to Open-Xml-PowerTools. It is actually provided in the Screen-Cast: Open Xml Sdk Version 2.6 at
https://www.youtube.com/watch?v=B13IYHKhcc8
The secret is, after doing
$ git clone https://github.com/OfficeDev/Open-XML-SDK
you should execute
$ cd Open-XML-SDK
$ git checkout master
$ cd ..
This adds the missing projects (especially the crucial System.IO.Packaging) which makes everything else work.