In attempting to load a second instance of Dimension Pro into Sonar 8.53 x64 I get the error message:
"Failed to insert soft synth.  The synth may not be properly installed."  I reinstalled Dimension Pro but continue to get the error message.  Can anyone help? 

You probably need to reinstall DP , but when you do, you must right click the .exe and choose "Run As Adminstrator"
 
If this doesn't work, please report back.
 
Also, please have a good read of this link:
 
http://www.cakewalk.com/Support/kb/reader.aspx/2007013238

This issue first started popping up back when Vista came along and with it the prototype for the ongoing annoyance called UAC that only got worse with Windows 7. To see if that's your problem, uninstall and re-install Dim Pro as Administrator, as suggested by Colin above. If that doesn't fix it, try running SONAR as Administrator (right-click on the sonarpdr icon and select "Run as Administrator"). If all is well when you run as Administrator (not just under a user profile with admin rights, but as "Administrator") then it's that pesky UAC that's getting in your way.

Thanks for the advice to install Dimension Pro as administrator.  That worked in that I can now load multiple instances of Dim Pro.  However, Sonar will not run when any of these instances are active (not archived).  I am going to try running Sonar as administrator as suggested and see if that makes a difference.

I tried running Sonar as administrator and this did not handle the problem.  Sonar will not run when Dimension Pro is loaded and active.  It will run when it is loaded and archived.  When it is active and you hit the space bar to start the song, the program freezes for about 20 seconds, not letting you do anything.  Then it will unfreeze, but still not play the song while the Dim Pro synth track is active. 
I am running Sonar 8.53 x64 under Windows 7 on an Intel Core i7 3770K processor.

Have you patched DimPro to v1.5? If not, you may want to do so. Leave the old DXi version installed for older projects but see if the newest VST version has the same problem. Patches are here. If you are still running a version older v1.2 you will need to apply the v1.2 patch first. I do not remember if 8.5 shipped with v1.2.

thank you scook, it seems to be working with Dim Pro 1.5!