When you are configuring SSL certificates for Exchange Server 2013, after you have generated the certificate request and received the SSL certificate from the certificate authority, you then need to complete the pending certificate request.
In the Exchange Administration Center navigate to Servers -> Certificates. Choose the server you are configuring the SSL certificate for and highlight the certificate that has a status of “Pending request”.
With the pending request highlighted click on the link to Complete.
Enter the UNC path to the certificate that you were issued by the certificate authority, and then click OK.
When the pending request is completed and you return to the main EAC window you will see the status has changed to “Valid”.
When the it is showing as valid you can then assign the SSL certificate to services on the Exchange 2013 server.