I had been dealing with an issue for months with third party plugins not loading in Sonar 4, and I never found the actual solution in any searches.  So, I'm posting this just to help anyone who may still be using older versions of Sonar that used the VST adapter program to load plugins.
 
The issue I was having was that all my third party plugins would not show up in Sonar, even though the VST adapter indicated they should be ready for use.  The solution is to run the VST Adapter as an administrator.  Right click on the VST adapter shortcut in the start menu and "run as administrator".  After doing this, your VST plugins should now appear in your plugins menu in Sonar.
 
You do not need to run Sonar as an administrator.  This was suggested by some, but didn't fix the problem.

thx Marob & welcome to the forum
 
martin

Thanks for the helpful info Marob. This might help someone in the future.
Im glad to hear Sonar 4 is still running for you on Windows 7. Have you considered upgrading (something like Home studio is 49.99 USA dollars) ?
 
Some people are content in using older versions and I am one who believes if your making good music on what you have, then why change anything. So all the power to you my friend.
The only reason I suggest the upgrade is because the price is cheap enough and upgrading allows you to use newer software plug ins and integrations that are not possible on older versions, thus moving your creative levels to a new place.