:)  Not exact, similar.  The content won't load because of a specific key it's looking for.  Just like the old PA9 installer was keyed to look for a specific DX version instead of =>.
 
Running the custom installer isn't too bad.  Hacking around the registry or completely reinstalling Dim Pro as potential workarounds is a bit much.
 
Having separate installers that don't look for magic numbers is the most sensible approach, IMO.

May want to contact Cakewalk technical support.

Did anybody resolve this? I just came across the same problem and copying the registry entries didn't fix it, sadly.

You can mark this as SOLVED!  I have been wondering what had happened to these sounds "Digital Sound Factory Classic Keys", along the path of X1 - X3 updates.
 
Sorry to drag up an old thread, but I ran across this issue today, searched up this thread and decided to have a go at it ...
 

scook
It may be looking for the old DimPro path
HKEY_LOCAL_MACHINE\Software\Cakewalk Music Software\Dimension Pro
instead of the new one
HKEY_LOCAL_MACHINE\Software\Cakewalk Music Software\dp
 
Copying the new path to the old one may work.

Scook was correct, but it took a little more fiddling to get the correct recipe. Here's what worked for me:
 
1. Locate the "dp" key in the registry (the new one that scook mentioned).
2. Right click on the key and click export (just the selected branch) name it & save to desktop as .reg file.
3. Open the new .reg file in a text editor.
4. Edit the line "[HKEY_LOCAL_MACHINE\SOFTWARE\Cakewalk Music Software\dp]" so that the key name appears as Dimension Pro, like this "[HKEY_LOCAL_MACHINE\SOFTWARE\Cakewalk Music Software\Dimension Pro]"
5. Save the .reg file as a new file, give it a new name.
6. Double click the new .reg file and it will install a new key for "Dimension Pro". Your working install of "dp" is not affected.
7. There is just one more thing to change in this new registry key.   You will need the old serial number from the Sonar 8 version of Dimension Pro.  Mine had a serial starting with CWDM110, on a yellow sticker on the back of the disk sleeve. I also have three versions of Dimension Pro 1.5, but those serials will not work. Apparently, the Additional Content installer looks for that old serial. Don't worry about the product key or the version number in the reg key.
8. Now you can run the Additional Content disk! You will get a menu where you can un-select anything you already have. I just checked the Digital Sound Factory keys, because I already have the latest Expansion Packs.
9. On my system the new Digital Sound Factory Programs content got dropped into C:\Program Files\Cakewalk\Dimension Pro\Programs. I had to move the new programs folder over to C:\Program Data\Cakewalk\Dimension Pro\Programs (where all of my Dim Pro programs are installed).
10. Finally, open the Dimension Pro Program Browser and hit F5 to refresh.  Enjoy!
 
Nice to have  "Digital Sound Factory Classic Keys" back :-)