Audio Units
Noteman says: See Using 3rd party VST/AU libraries with Finale and PrintMusic to learn how to set up Audio Units.
Audio Units (AU) are interfaces for integrating software audio synthesizer plug-ins such as Finale's included ARIA Player for Garritan Instruments for Finale sounds. Finale supports all Audio Units instrument plug-ins. Audio Units and similar technologies use Digital Signal Processing to simulate traditional recording studio hardware with software. Thousands of plug-ins exist, both commercial and freeware.
You can assign instruments and effects included in Audio Units plug-ins to Finale channels using the Audio Units Banks & Effects dialog box. This includes user-made instrument and effect presets created in like GarageBand and Logic.
See also:
Configuring instrument playback
Editing banks and players