The preferences menu will also let you set other elements of the plugin, like whether you want find as you type or not. You can also configure your default search channel, if you want a channel image or not, if search results should open in a tab and many more.
AcidSearch itself is really easy to use. Clicking on the little arrow in your search bar will reveal your menu of different search channels. Just select the one you wish to use to make it active.
AciSearch is the ideal companion to the Safari search bar, which I personally feel is a bit limited.