@stefl, on "Add Search Engine" is available when the keyboard is hidden. Unfortunately on duckduckgo.com it activates automatically, what probably caused you problems. So deactivate keyboard with the button in lower right corner and then long tap on the search field, you'll have "Add Search Engine" option. Then when you activate address bar, click on the Google icon over keyboard to change to DuckDuckGo. Unfortunately this setting is reset to Google when you restart the browser.

@mbaluta will the reset to google ever be an option to disable? I want DuckDuckGo as my default engine and it's a pain to reset each time. Nearly pushing me to stop using opera mini.