Cannot Connect to Exchange Account - Certificate Issue?

I have exchange account and I need to install a trusted root certificate. I have the certificate and have installed it in my login keychain and also the system keychain. Unfortunately, under the Advanced settings, Security tab there are three areas to select a certificate. When I click the dropdowns, however, the option to choose a certificate is grayed out. I have searched online to no avail. I suspect that the dropdown I need is Certificate Authentication - Client Certificate. The only option to select in the dropdown is "None Selected." "Choose a certificate" is grayed out. How do I tell this account to use the certificate I have to make Outlook connect to Exchange?