There are a couple options for integrating a subscribe form into your WordPress site:
You can use our free Direct Mail Subscribe Form WordPress plug-in to add a simple subscribe form to your blog or website. Click here for installation instructions.
Our friends at ARScode have integrated Direct Mail support into their excellent Ninja Popups plug-in. Ninja Popups gives you great looking themes, lots of customization options, statistic tracking, and more. Check out their site for more information.