Settings > Manage search engines > click 'Make default' on the provider in the 'Default search engines' list you wish to use. Currently you can only use search providers in that list, not ones you've added yourself. DuckDuckGo is in that list though.
@joshl, why do you keep telling people you can't change the default search engine? You can, its just the selection is limited. I especially wouldn't say 'you can't if the one he wants to change it to is in the list of ones you can change it to.