Fired up IE9 to take a look.

One click on the search icon in the address bar and Bing and Ask.com (really?) are listed as two default search providers.

There is an Add button which takes me to an Internet Explorer Add-On page. DDG and Google are highlighted as popular options. Click on the "tile" and another click to install.

So 4 clicks for IE9.

It's 4 clicks in Chrome (Menu - Settings - Settings - Manage Search Providers, to get to the "Manage Search Engines" screen but DDG is nowhere to be found. YOu have to manually add it and it isn't very clear how to do it because the textbox is truncated for the actual url.