Ok, whatever will be that, this feature only works in latest versions of Chrome, Firefox, IE8 and Safari (Mac). For Opera users, here is a workaround to enable Google Instant.
Visit www. google.com in Opera browser. Note that, currently Google Instant is available in US, UK, France, Germany, Italy, Spain and Russia, if you are on other countries click this http://www.google.com/webhp?sclient=psy link in Opera.
Then, right click anywhere on the page and select "Edit Site Preferences". Go to the "Network" tab and choose "Identify as Firefox" from the drop down list on the bottom. Click OK to save. [Via]