Thank you for your time to have a look at my problem.
It seems like that I found the solution now.
I can get the medoids now.
I added getMedoids(), setMedoids(String medoids) methods in FlatCrispClusterModel, FlatClusterModel and KMedoids.
Also a small modification in KMedoids.
the forthcoming release 4.3 includes a new version of the kmedoids algorithm. It then will provide a simple model, providing the medoids.
This means: Your problem will be solved then. But this means your changes are somehow outdated yet
By the way: If you want to test the developer version of 4.3, check out the branch Zaniah. There are some other improvements regarding the clustering, espacially the performance.