You can install the ARIA for Acid plug-in by following these instructions from Sony's support website: http://www.sonycreativesoftware.com/support/files/InstallPlugIns_ARIA.pdf
The ARIA for Acid plug-in, like the Garritan ARIA Player, requires activation. You can download your activation keycard by logging in at here. You may have also received it by email. Simply reuse the same keycard by dragging it onto the ARIA Player.
Contact Sony Creative Software with any support questions for this product.