There is no such restriction.
I am running Sonar8 Studio, as well as the other versions listed in my signature. I can insert DimPro in any of them.
I did just discover this, though.
If the synth is not enabled as a plug-in in the plug-in manager, it will not appear in the selection list in your project.
Highlight DimPro in the plug-in manager.
The Plug-in Properties button should activate when you do this. Click it.
In the properties window that appears, make sure "Enable as plug-in" is checked. Click OK.
If I uncheck this box, I can duplicate what you are seeing.
Be sure that "Configure as synth" is checked while you are at it.
Let us know if that works.