Solutions: In SONAR go to Options | Global, VST Plugins tab. You'll need to then hit the Add button, and browse to the location where the VST component for your Aria player is installed. Add the directory where the file is located, then hit the Scan VST Folders button in SONAR.
I can not find Aria inside program files/garritan.
If you had sonar installed already when you installed GPO then check the C:\program files\cakewalk\vstplugins folder for it. If it's there but not present in the menus click Rescan on the sonar plugin manager. The plugin manager does not display which plugins are present while you browse directories, but will update once you add and click on Rescan.
If you can't locate it there, you should be able to find a copy of the dll files in C:\Program Files\Garritan\Aria Player\VST
Aria Player VST gives a stereo pair of outputs (summing its 16 channels internally), while the Aria Player Multi VST has 32 distinct outputs (giving you access to all 16 instruments in stereo).