Creating a Certificate on Exchange IIS

This question might be in the wrong venue but you have to start somewhere.

I'm attempting to configure OWA using SSL on my Exchange 2003 w/SP2. To to
this I have to create a new certificate via IIS 6. Now when the part of this
procedure comes to choosing a certificate authority the CA, whichI have
running as an enterprise CA on a domain controller, does not show up in the
drop down window. Why? It will see other CAs but not the one one I want to
use?