Pentadactyl with DuckDuckGo as default search engine

I'd like to set DuckDuckGo as Pentadactyl's default search engine. I added DuckDuckGo's query URL to about:config's keyword.URL variable. I also installed DuckDuckGo's engine via "Add to Firefox" link.

What else can I do?

First, you need to install the search engine at:
https://addons.mozilla.org/en-US/firefox/addon/duckduckgo-ssl/

Then you can set the default search engine using:

set defsearch=duckduckgo

From the help file:

'ds''defsearch' string(default: google)

Sets the default search engine. The default search engine is used by :open and related commands for arguments which include no search or bookmark keywords and can't otherwise be converted into URLs or existing file names.

This means that with 'defsearch' set to youtube, :open Tim Minchin behaves exactly as :open youtube Tim Minchin, so long as you don't have a search or bookmark keyword called ‘Tim’.