How to make Duckduckgo default search engine?

Better off with Google, Yahoo or Bing as the default search engine.

Best.

If you want to fix your Google search to a specific Google location, or a specific alternative search engine, I recommend that you download and install the free plug-in Glims:

http://www.machangout.com/

which not only does that but much more equally useful stuff! Such as enabling Safari to run full-screen, and more.)

(It adds items to Safari Preferences).

Not only does it already list a number of search engines and regions, but you can also add any other search engine not already listed in Glims by using the method described here:

http://www.machangout.com/tutorials/addsearchengine

Just enter the name you give the search engine in the name field, and the URL for it in the Query URL field.

the ddg extension doesn't do that, as they say here

✗ Search DuckDuckGo using the browser's search bar (Safari does not allow --see alternatives below).

You may have discovered this already. There is an extension that should work located here:

http://safariextendr.com/extension/duckduckgo#.UpvuRJHI921