If you installed your Garritan product elsewhere than the default location, you may suffer from a known bug where the .ariax files are misplaced.

To solve this problem, go to your chosen install location (ex: "User Specified Location/My Garritan Library/". You'll find the following path: "presets/com.Plogue.Aria"/.

Move the contents (.ariax files) to "Application/Garritan ARIA Player/Presets/com.Plogue.Aria/*The name of your product*).

The ensembles will be visible the next time you start ARIA Player.