scook
Also need to make sure the appropriate plug-in format is installed. If the project is old enough, it may require the DirextX version of DimLE. Installing DimLE from the Cakewalk product which came with the plug-in is the best bet.

Yes, perhaps that's the issue (although it doesn't explain why the DimLE DLL I have doesn't show up in the plugin manager at all.)

Was the dll installed or copied into the folder? If copied it could be the 32bit DirectX plug-in or regsvr32 needs to be run. If installed, maybe regsvr32 failed.

 
Kylotan, you could buy a Digital Sound Factory sample library that has DimPro LE included for free.
 
The current cheapest looks to be the Volume 1 Yamaha Grand Piano at 20$.
 
EDIT :  I see that you already have some version of LE so you're problem may not be as simple as just installing a fresher version.

I dug out the Sonar 7 disk, installed Rapture LE and Dimension LE. Rapture LE now works fine in Platinum (well, apart from hanging roughly every 1 in 3 times I change the preset, but hey), but Dimension LE didn't show up in the Plugin Manager at all. But then I installed the 1.5 update - turns out the DimPro installer has a dropdown box letting you choose DimLE if you go 3 or 4 screens in - and now it shows up in the Plugin Manager. It spit out some content files somewhere on my hard drive without my permission, but I guess you have to take the rough with the smooth.
 
Now to see if I can update Rapture LE and get that behaving something close to tolerably.