How to Use the Postmatic Widget
In addition to personally inviting people to subscribing to your blog through Postmatic's invitation system, you can also place a widget on your blog where people can sign up on their own.
From your dashboard go to Appearance > Widgets. On the widgets screen, under Available Widgets, you’ll find a new widget - Postmatic Subscribe. Drag the Postmatic Subscribe widget to your desired sidebar. You can choose the title for your widget, as well as whether or not you wish to collect names along with emails. When done, click on Save.
Below, you can see how the widget will appear on your blog.