1) Launch Sonar, go to ‘Edit -> Preferences -> File -> VST settings’
2) Make sure the correct location is added to the VST Scan Paths. The default locations are as below:
32-bits - C:\Program Files (x86)\VstPlugins
64-bits - C:\Program Files\VstPlugins
3) If the steps above are not working, try the Scan option to re-scan the target folders.