In order to utilize Garritan Libraries in Cubase, verify that the proper ARIA Player Components are installed in the VST Plug-In folder to which your host program is pointing. In Cubase for Windows, this folder is located in C:\Program Files\Steinberg\Cubase 6\VSTPlugins. On Macintosh, this folder is located in Macintosh HD > Library > Audio > Plug-Ins > VST.
Verify that the ARIA Player Mutli VST.dll (Windows) or ARIA Player VST Multi.vst (Macintosh) file has been copied to this folder. If it is not present here on Windows, you can copy it from the following location:
Windows: C:\Program Files\Garritan\ARIA Player\VST
You can then update the Plug-in list in Cubase by going to the Devices Menu > Plug-In Information. Click the "Update" button here. The ARIA Player will now be listed as an available Plug-In.