Thanks for your help! Your reminder to check the object path brought me to the solution: I still had declared the Poser6 directory in the import option and since I had to install V4.1 in the Poser7 native directory (and not in an external runtime like V4.0) Vue didn't find it. 
The problem is that you need Version 4.1 if you want to use the muscle morphs (and I guess the creature morphs and probably all morphs and additions that DAZ is going to publish over the next years!). 
Thanks again!

Sebastian