As adding of it is mostly same as like adding of retweet button to each blogger posts.
As we can add button in two ways one is to make it float to the beggining of posts to right or left where ever you want and second is to show it at the end of posts when it ends.Both demo pictures you can see below.
Now to add digg button like this just above in picture Follow these steps.
(a) Follow this Method If you are not using read more hack.
Login To Blogger Go to Layout > Edit Html > Expand Widgets > Press Ctrl+F and find for below codes
And replace them with below codes.
*Note:-If you dont find above code you find this one