Add a digital signature to macro project

Reference:
http://office.microsoft.com/en-us/project-help/add-a-digital-signature-to-a-file-or-macro-project-HP005249557.aspx

Steps

1. 用 Alt + F11 叫出 Visual Basic Editor.
2. On the Tools menu, click Digital Signature
3. 選擇要用的憑證