Exchange 2007 Digital Signature S/MIME for OWA

We have exchange 2007 SP3 on an old server 2003R2. The users are wanting to start sending digital signatures (encrypt their outgoing email to their clients). Right now I just want to focus on the OWA users. So all my users access the OWA by going to https://remote.mycompany.com/owa (where remote.mycompany.com is the same on the Certificate). When I go to send an email with it digitaly signed I get the following error.

I wanted to check the Certificate being used so i had to enable a registry entry...
Expanded  HKLM\System\CurrentControlSet\services\MSExchangeOWA\SMIME
Right clicked the SMIME key and click New and click DWORD (32-bit)
Named the new DWORD value AllowUserChoiceOfSigningCertificate
Double clicedk AllowUserChoiceOfSigningCertificate and set the value to 1.

I click options > email security > click manually pick the certificate it says none and when i click the "choose signing certificate..." there is nothing to choose its totally blank.