In the library manager interface you need to open the Padstack Editor & Cell Editor separately to create the new padstacks and cells and then associate them to a part in the Part Editor.
If you launch library manager from xDX Designer/DxDesigner only a sub-set of the editors are available and if you open it from Xpedition a different sub-set is available.
Thank you very much. Now everything is as it should be. By the way: Very quick answering. Big thanks