Okay. I took the plunge and went Win-7 64-bit. I did a clean install of Sonar 8.5.3 on a new machine. All other plug-ins that came with 8.5 appear to load and work, except for Pentagon-1. I get:
 
    Failed to insert soft synth.
    The synth may not be properly installed.
 
I tried re-installing 8.5 because of this and it failed after both installs. Is this a sign of an incompatible plug-in under 64-bit?

(bump) Sorry!
 
Somebody must be running 64-bit and 8.5 and can confirm this or not on their system?
 
Much appreciated. Thanks.

It's working fine here in Windows 7 x64 on SONAR 8.53.  I verified it in both the 32-bit and 64-bit versions of SONAR. 

I'm trying to think of some ideas on what it could be, but I'm a little stumped.  Maybe bumping this up will trigger someone else's memory.  It's a DXi synth, so it may not have registered properly with Windows when you installed it.  First verify that it shows in the DXi list in plug-in manager and doesn't show up under VSTi's.

Genghis
It's a DXi synth, so it may not have registered properly with Windows when you installed it.  First verify that it shows in the DXi list in plug-in manager and doesn't show up under VSTi's.

Yes. It is under DXI listed as: rgc:audio Pentagon-1
 
Hmmm. At least it works for you. I will try Cakewalk Support unless someone provides a solution.

Yeah, that's how it's listed for me as well.  Good luck and I hope they get you sort out!

Cakewalk promptly responded to my tech support inquiry and asked me to launch Sonar in Administrator mode (i.e., right click on Sonar launch icon and select 'Run As Administrator'). That seemed to do the trick. Pentagon started with no problems.
 
Now how can I get rid of this Administrator stuff? I don't want to have to do this right-click exercise everytime I launch Sonar.

Ahhh... should have known that was it.  I think there is a way to set up the shortcut so that it will always run in Administrator mode.  I believe you right-click the shortcut and look for an Advanced tab or button or something like that.

I don't worry about it because I disabled UAC on my DAW and that takes care of most of the Administrator type issues.  Some are hesitant to do that, but I only use that computer for making music, so there's not much danger in doing that.  Some don't recommend it if you also use the computer for surfing the Internet and email, etc.
If you want to go that route, go to Control Panel, User Accounts, User Account Control and slide the slider all the way down.

Genghis,
 
You got it. Right-click -> Properties -> Advanced. Simple enough.
 
Thanks!

Same here.  Thanks for the tip.

That gets the plug loaded for me, but whenever I try to route pentagon as a vocoder, it kills my audio engine within 2 minutes. 

Haven't been able to get that resolved. I'm hoping to try the vst version and see if that works better.