How can I add this search engine?

You can download a plugin from startpage.com, that at least works in Firefox and Seamonkey. It may, however, not work in K-M except with the help of a macro.

However, according to startpage's Q and A, their search engine is really one and the same as Ixquick Metasearch, which is already included in K-M's default search engine list. Just add Ixquick to your search menu by going to Tools > Web Search > Configure > Web Search.

Thank you, IXquick is the same engine.