Use the following preference in prefs.js
user_pref("kmeleon.general.searchEngine", "
http://www.google.com/search?q="
Change from google to the search engine you would like to use. Note that it needs to be in the format required by the search engine. Do a search and copy the url, removing the search string. The search string will be added to the end, so this field needs to be at the end of the url when it is entered above.