First, verify that the proper ARIA Player components are installed in the Audio Units or VST plug-in folder that Reaper uses. To find the selected folder, go to Reaper's Options Menu > Preferences and choose VST under the Plug-Ins category on the left.
Verify that the ARIA Player VST.dll (Windows) or ARIA Player VST.vst (Macintosh) file has been copied to this folder. If it is not present here, you can copy it from the following locations:
Macintosh: Macintosh HD > Library > Audio > Plug-Ins > VST
Windows: C:\Program Files\Garritan\ARIA Player\VST
Be sure to click "Re-scan" in the VST preferences after moving the files. Once the files are in the proper place, the ARIA Player will be available as a plug-in.