That's the second time now that I have found interface areas in the gui admin console to configure something that does not work at all. Manually entering those settings on the global or domain level would never allow the Check the Settings button to pass. However, prior to your response, I found the same commands for configuring Interop via the CLI here.
I executed these commands using the same information I was trying to enter in the GUI, went back into the GUI, it looked exactly the same as it did before but this time the Check the Settings button passed. So what is the commands doing that the GUI interface is not doing? Definitely a bug.
# zmprov mcf zimbraFreebusyExchangeAuthUsername zimbra
# zmprov mcf zimbraFreebusyExchangeAuthPassword changeme
# zmprov mcf zimbraFreebusyExchangeAuthScheme basic
# zmprov mcf zimbraFreebusyExchangeURL http://exchange.mycompany.com
# zmprov mcf zimbraFreebusyExchangeUserOrg "/o=First Organization/ou=First Administrative Group"