To use it check settings in Safari Preferences on Extensions tab. Choose you favorite search engine (Yandex is preselected by default, as my company :)). This is Default Search Engine, it will be used in any time you typed simple text into Location Bar.
But sometimes you need to search over Facebook, Wikipedia or WolframAlpha, not by your selected Default Search Engine. To do this just prefix search query by shortcut:
?is your Default Search Engine selected in Safari Extensions Settings.
For example, if your Default Search Engine it not Wikipedia, then you can type 'w Antoine de Saint-Exupéry' in Location Bar and Safari open Wikipedia page about one greatest writer from Marseille.
If you using shortcut, then OmniBar will think what you not needed to use your default engine, so, it will exclude Default Search Engine and will search for shortcut in all other.
Since Safari 6 have omnibar on board, this extension maybe obsolete and you can remove it (especially you not need custom Search Engines). But it still can be used if you requires to use non standard (Google, Yahoo, Bing) search engines such as DuckDuckGo or Wikipedia.
Since Safari 6 API this extension work pretty well and all bugs with wrong decoding of search
queries from URL has gone. This extension intercept
beforeSearch event and do not try to decode
anything from url (as for Safari 5).
Safari 5 has no ability to use
beforeSearch event, it used
beforeNavigate event and this make some restrictions
to OmniBar extension. Sometimes it can't handle correctly some queries you typed in location bar. Especialy if it
contains such symbols as
, etc. Safari force HTTP error before any events sent to OmniBar extension. This
best way to fix it – is using modern Safari 6.
Simple go into Safari menu, then Preferences, select tab Exceptions select OmniBar and click Uninstall button.