Is using OS X v.10.5.8

I'm trying to load Garritan World Instruments to add to Finale with little success.

While attempting to resolve the issue with Garritan, they suggested I set up another log on account and attempt loading the software from there as some sort of test. I set up the "new" account and began the install procedures. But when when I got to the point of entering the my new account name and it's related password it was not recognized - message: "You must type in administrators name and password to make changes to installer". Garritan is claiming it's a hardware issue. I'm frustrated. Can you help?