I did a clean install of X3 on my machine several months back; I went to insert Pentagon as a softsynth, and I get the message that it is not there or installed improperly.
Um . . . that's weird. Anyone know what's up with this?

I believe you have to Run As Admin to get it working.  It is certainly part of X3 Studio/Producer.
 
If you launch Sonar with Run As Admin and it is still not working, then rescan your plugins.  If THAT still doesn't do it, you might want to make sure you install Sonar X3 with Run As Admin.
 
Please note that Run As Admin is NOT the same as logging on with a user account that has Admin privileges.  Pentagon seems to require Sonar running with Run As Admin, and may need it for the Sonar install, as well.
 
Bob Bone
 

this guy had the same problem http://forum.cakewalk.com/FindPost/2920431 with the same solution

Boo . . . this has tripped me up more than once.

Phooey - this is inconvenient.
I have tried uninstall and re-install with X3 - it made no difference: apparently once you install just by double-clicking (um, which is the normal way to do these things - are you listening Cakewalk??), you can't un-do this without major registry surgery.
No thanks. I guess I'll just have to wait until I get my next computer, and then hopefully I will remember that non-intuitive trick of "Run as Admin" and install it correctly.
Cakewalk - please fix!!

I am a bit confused.
 
1.  Did you right-click on Sonar's launch icon and then click Run As Administrator, followed by rescanning your plugins?
 
2.  You can change the default run-mode options for Sonar to always use Run As Administrator, by right-clicking on the Sonar launch icon, then clicking Properties, then clicking Advanced, then checking the box for Run As Administrator, then click OK, then click Apply (and OK it if it wants an additional OK to continue).  This is how I always run Sonar - it's a one-time change to the Sonar icon.
 
3.  If the above does not work for you, and you choose to uninstall Sonar and install it again, right-click the Sonar installer program and click Run As Administrator.  You will still need to Run As Administrator once you finish the install, using option 1 or 2 above.
 
4.  If you need further assistance with the above, please post back with any other questions, and we will collectively try to assist further.  If you need assistance with a clean uninstall and removal of Sonar, please also post that and I will supply the steps.
 
Hope that helps, 
 
Bob Bone