To add a Digg button a post you simply have to put a piece of code in the post.
The code looks like this:

The finished result should look like this:
This button only works if you have submitted you article to Digg.
Once you have added the code and submitted your article, anyone who has a Digg account and is logged in will be able to easily Digg your post.
Feel free to see how this button works by using this Digging this post. ;)
If you are having trouble making this work use this step-by-step FAQ complete with screenshoots. Or you might want to view the offical WordPress announcement. Enjoy!
source: natedesmond.wordpress
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