Just remember I never responded to this thread but I did jump through some Cisco hoops months ago to find out some not so great info.  I reached out to my account rep and after going through multiple groups they hit some group in India who basically ...

Same issue with it not working on any browser I've tried.  I even tried to get in contact with someone at Cisco but they kept asking for a support contract like I was trying to open a case for my equipment/software and not for a tool on their site.  ...

You have to first go under Cisco Unity Connection Serviceability web portal.  Then you can access using the above method "Tools > Reports > Unused Voice Mail Accounts Report"