to your template’s php file before or after the line <?php the_content('something'); ?> depending on if you want it to appear before or after the posts.
If you want to submit to Digg there’s a great WordPress plugin called Digg This for WordPress Reloaded that lets you to add a very cool “Digg This button” like the one I have in the posts, without changing the template’s codes. It also comes with a Complete admin interface with options for many customizations, like positioning the button, what pages to show the button, the style of the button , and many others.