To import a digital certificate to Windows Mail follow these steps:
1. Open Windows mail and click Tools > Options.
2. Click the security tab.
3. Click the Digital IDs button.
4. Click Import, then on the next screen click Next.
5. Type the path or click Browse… to find the digital certificate file you wish to import into Windows Mail. Click Next when done.
6. Choose where to place the certificate. Usually you do not change this from the default option of Place all certificates in the Private store. Click Next when you are done.
7. Click Finish then OK on the popup dialog.
For detailed instructions on using digital IDs to sign or encrypt Windows Mail messages refer to Microsoft article: "Using digital IDs to sign or encrypt Windows Mail messages " :