Pick a text file and use Ctrl-I to change the ‘open with’ application.

It is probably the easiest way.

More info in

http://superuser.com/questions/231854/default-editor-for-files-without-file-name-extension-in-mac-os-x

Before Ctrl-I, I actually tried to open ~/Library/Preferences/com.apple.LaunchServices.plist. It opens up XCode4 which is actually not an ideal tool to edit a plist.