Wikipedia:Google Custom Search
Here are the required steps for you to use the Custom Search skin:
Make sure to save your changes (you may have to type in a CAPTCHA to successfully submit changes). Reload your current page in your browser (Hold down Shift while clicking Reload), so the above changes take effect.
Your search box should now be updated. When you search, results will show up inline in the current page, instead of opening up a new window, or tab. The appearance of the search box will depend on the skin you are using (Vector or Monobook), as shown in the screenshots. The appearance could also change depending on the web browser you are using.
Remember that in order to enable this new contextual search experience, you need a user account and need to log in to Wikipedia. When you search, your results will show up inline at the top of the Wikipedia page you are viewing. You can click on the relevant tab to toggle between all Wikipedia matches, just the matching Wikipedia pages linked from the current page, or external (non-Wikipedia) linked pages. If a Wikipedia page does not have a lot of content or links, it is possible that there are only a few linked results.