Well, I'm not familiar with the "add search engines" you name--can't recall seeing that on Firefox, on no Linux distro and not on Windows either?
What you can do: click the down arror icon in the search bar, select "manage search engines", click "get more search engines" (which takes you to http://www.linuxmint.com/searchengines.php) and you can click on the various search engines listed here for your convenience. The details page for each search engine has instructions how to add it. Some of these are partners of Linux Mint and adding them this way might share any revenue the search engine makes with the Linux Mint project.
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