If you're using Chrome, follow the instructions here.
If you're using Opera 9.10 or below, you need to install and run Opsed. Once that's done, restart Opera and click here. Restart Opera again and it should be available.
If you're using IE 7, just go to WikiFur and look in the search box dropdown, then select to add it to your search providers (don't just select it in the main list, otherwise it won't be saved for later).