An ARIA update a little while back accidentally broke some coding in IO, causing some instruments to come up missing when the user tried to load them.
The MakeMusic support staff has been providing updated installers to IO users who contacted them about the issue, but now the update is available for all IO users directly online.
Erin V. from MakeMusic Support sent me a message this afternoon which included this information:
"...We have now updated the installers for Instant Orchestra. Any new customers who purchase the product will receive the updated installer which will work with the new ARIA engine, and any current users can re-download the program from their account at garritan.com..."