Hi Norbert,
It's a little weird how you have to do this, I suspect as a consequence of moving to the "Application-Center" organization of the AdminStudio 10 catalog.
Basically, you have to right click the other folder and select "New Application" to create a container to ctrl-drag the existing MSI package to from under its Application container. Dragging the Application-level object doesn't work.
Does this work for you?