It would seem that Digg disabled this feature and now you can no longer auto submit articles even though you can add your RSS feed it won’t auto submit them into Digg, below is the original article from when this feature worked for reference only.
I found that the new Digg allows you to configure it to read in your blog RSS feed and auto submit all new articles to Digg automatically without you having to submit your content manually.
All you have to do is login to Digg and click on your profile picture and select Profile:
After selecting your profile you will then click on the Import Feeds button on the left and enter your Blog RSS feed URL.
After this you will be asked to cut/paste some information into your RSS Feed body or inside a post (which gets picked up by the RSS Feed).
Once you verify the feed your new content will always be automatically submitted to Digg and you can start promoting it to your networks.
Then all you have to do is sit back and check your digg profile to see that your imported feeds get submitted automatically, you will find them listed in the bottom right of your public profile page (digg.com/username)
If you are on Digg why not follow DragonBlogger on Digg while you are there!