What should I do when my Window 8 wants a digital certificate in order to add an exhcange account?

I am the system admin for my company and I'm trying to add a user's exchange email to his new tablet which runs the Windows 8 OS.  I know i'm entering all the correct information (server name, domain, username, password) but each time I click connect I receive a message saying that I need  to install a digital certificate on the PC. Contact your system administrator for more info.  What am I doing wrong?