If you're using AdSense dead easy - just insert an AdSense widget and click the appropriate check boxes. If you're using something else you'll have to embed the ad code at the bottom of each post - just click the HTML tab on the compose window and paste your ad code wherever you want it within the body (or at the foot) of your post.
Go to your Dashboard. Click Settings. Then Template>Page Elements. Click the "Edit" button on the "Blog Posts" element in the editor. It's the last checkbox under "Select Items" called "Show Ads Between Posts" Click it, save your changes, and you're done! Easy as pie.