I have the same problem, but with E-MU Proteus-Mo' Phatt.
System: Win 7 64-bit, Sonar X1 64-bit, Dim Pro 1.5 64-bit on Dell Precision M4700 i7.
Symphonic HD strings runs fine, but DP won't find the Mo' Phatt sounds. They don't show up in DP after refreshing with F5. I've followed your instructions above and also copied the folders to my default DP multisamples folder on my D-drive. Nothing seems to work. The sounds just wont show up in DP.
The odd thing is that the Mo' Phatt installation creates a \Programs-folder and I can't find any other Dimension Pro Programs-folders anywhere in my system. Still DP runs fine and I can find and play all other sounds in DP apart from the Mo' Phatt library.
Another odd thing is that the Registry entry you refer to above doesn't include any reference to any multisamples folder on my system. However HKEY_LOCAL_MACHINE\SOFTWARE\Cakewalk Music Software\dp does include the correct path to my multisamples folder D:\WaveFront\Ljud\Cakewalk\Dimension Pro
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I've found a solution now and naturally I failed to see that the programs folder should be located in the hidden ProgramData folder. When I copied the Mo' Phatt library programs to this location everything worked fine.
However, I think the Dimension Pro installation procedure and its libraries need a serious overhall as it shouldn't be this difficult to install a few new sounds.