We are sorry for your inconvenience,
I also have this issue.
It is not even possible to create an empty Presantation with
PresentationEx presentation = new PresentationEx();
That way the library is unusable at the moment.
Went back to 3.0.0 which does just fine.
Btw.: What is the HUGE Version Jump 3.0.0 -> 6.7.0 for?
I do have the exact same issue - the slides library seems unusable for PPTX with 6.7.0 Version. My environment is linux (ubuntu 12.10) with java 1.7.0_07 64bit