If you're using Mozilla Firefox, go to Settings > Add-ons and click the Enable button next to Do Not Track Plus Add-on. After that, close the browser and open it again.
In Google Chrome, go to Options > Tools > Extensions, check the Enable box next to the toolbar and restart the browser.
If you have Internet Explorer as the default browser, go to Tools > Manage add-ons > Toolbars and Extensions, select the Do Not Track Plus Add-on and click the Enable option.
In Opera, press Ctrl + Shift + E key combination and click the same Enable option.
If it doesn't work, temporarily disable your antivirus and reinstall it.