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As you’ve probably noticed, 2011 works for the most part except when it doesn’t. Certain actions may make it crash. In fact, it still works only because you installed and authorized it in Mavericks (or earlier) originally. It will work the same if you upgrade to Sierra but not at all if you go to High Sierra. 

Hi, My name is Alison.  I need your help !!! 
I installed Garritan sounds but they do not play through Finale even though I selected the"Midi/Audio and Audio Units Banks and Effects and Aria Player. I restarted the application and restarted my computer after installing Finale 25, and after installing Garritan Jazz plugin. I followed all the instructions but the Garitan sounds just do not play though my new downloaded copy of Finale 25. The only sounds or instruments that do play are the ones that come with Finale 25. I hope you can help.

Let's start with the basics:

The purchased download contains 2 installers — did you run both? If you enabled the demo, it does not come with the Aria/Garritan installer that Finale needs to set the paths correctly. If that's the case, log into your MM User Account, download and reinstall. Then reinstall Garritan Jazz.

Did you reboot? You don't give your OS but Windows and Mac High Sierra seem to require it.

If that doesn't do it, you can contact Tech Support through the Submit a request link at the top of the page.

Alison,

1. Have you selected the Play Finale Through VST option in the MIDI/Audio menu. Although Garritan sounds respond to MIDI codes, they are not considered MIDI sounds but are considered VST instruments.
2. When you look at the Aria Player, has it been populated with instrument sounds or is it empty?
3. When you installed the Jazz package, did you drag the license image onto the Aria player to activate it for the Jazz package?

Just tossing out ideas in case Mike H and I interpreted your problem differently.

I agree ... we really don't have enough info to give an informed answer.

BTW, if you're on a Mac, substitute AU for VST.