It will prompt you to select the private key you want to use to sign the mail, and your passphrase.
When you open an existing mail, you will notice a new ribbon group, Outlook Gnu PG, with a Verify and Decrypt button.
No HTML mail or attachments, that's for a future version :) To install the addin, start by adding your Trusted Sites (you can do this in Internet Explorer - Tools - Internet Options - Security - Trusted Sites - Sites).
Download the Outlook Gnu PG Click Once installer and execute it. When the addin is installed, you can open Outlook and a Settings dialog will show up. and select the directory where your the you previously installed is located.
Simply click Verify to check if a signed mail is valid or not. Likewise, click Decrypt to decrypt an encrypted email.
This will ask you for your private key to decrypt the message with.