The shema extension is not the problem. I can see the attributes with the ADSI-Editor an can populate them. It seems to be that the actual release of the gui-extensions will not work on x64 systems. I installed two servers for testing, one server2008_x64 and one server2008_x86. On the x86 system everything works like expected. On the x64 system the same problems occured as on the server in my first posting.
I hope this bug will be fixed soon, because it makes not much fun to populate the attributes of users with the ADSI-Editor.