Hi Stefan,
I'm an Ivory user as well running version 1.72 on Windows 7 and it seems to be working fine here running VE PRO Build 11262 under the 64 bit Server mode.
When I go to add plug-in, Ivory shows up here. Are you certain that your Ivory.dll file is stored where VE PRO is looking? My plug-in is stored under program files/synthogy/vstplugins/ so I added that unique path in the VST Settings window and then did a 'rescan all' to ensure that it found it.
Another possibility is that you are trying this on the Vienna Server 32 bit mode instead of the 64 bit one.