Open Keychain access, click on the My Certificates category and your certificate should be in the list shown. Just right-click and export and you're finished!
If you used Firefox on a Mac, the export instructions are the same as they are for Windows (see that tutorial here).
Please note -- these instructions are for Mac OSX ONLY.
If you're using Safari on Windows, follow these instructions : http://ksoftware.freshdesk.com/solution/articles/17157-how-do-i-export-my-code-signing-certificate-from-internet-explorer-chrome-or-safari-on-windows-