Keychain and Mail


I just got my first Mac, a G4 OSX Tiger. I have been using a PC for many years. I know how to do all of this on my PC, but not on the Mac.

I am trying to sign my e-mails with a certificate. I have Mail setup with my POP3 account and that works. I have signed into Thawte to get my free certificate. I have the *.cer file on the Mac, and Keychain sees it, but how do I get Mail to sign my e-mails with that certificate. I think I am putting the certificate in the wrong folder in KeyChain.
From Mail's Help Menu;

Certificates are stored in Keychain Access on your computer.

To import a personal certificate:
Double-click the certificate file you receive from the certification authority. Keychain Access opens and imports the certificate.

For Keychain Access to recognize a certificate file, it must have a file extension that indicates it contains certificates. Some examples of file extensions are ".cer," ".crt," ".p12," and ".p7c."

If Keychain Access does not import the certificate, drag the file onto the Keychain Access application icon, located in Applications/Utilities.

If you still can't import a certificate into Keychain Access, the certificate may have expired or the certificate may not be valid. Check with the certificatation authority.

You need a certificate for every email address you are using. If you want to use the certificate on another computer, find and copy the original certificate you downloaded from the certification authority and move it to the other computer. Double-click the certificate file to open Keychain Access and import the certificate to that computer.

I have read that. The *.cer is imported in Keychain and labeled as Valid.

But Mail does not sign my e-mail with it.

"Choose File > New Message. In the Account pop-up menu, choose the account for which you have a personal certificate installed in your keychain."

When I choose File> New Message I do not have a pop-up menu displaying multiple accounts, I am assuming this is because I only have the one account in Mail. I proceed by selecting New Message, and the new message comes up, but there is no icon indicating that the message will be signed or encrypted.

I keep thinking that I need to specify the e-mail account in Mail with the certificate in Keychain, but I can not find where I would do it.