Please follow the steps below:
1) Launch Sonar, go to ‘Edit -> Preferences -> File -> VST settings’
2) Make sure the correct location is added to the VST Scan Paths. The location of plug-in files are depend your setting during installation.
The default locations are as below:
32bits - C:\Program Files (x86)\VstPlugins
64bits - C:\Program Files\VstPlugins
3) If the steps above are not working, try the scan button on in the Scan Option to re-scan the target folders.