If you're using the Layouts-enabled template in Blogger, you can add an AdSense for search box to your blog by following these steps:
- Sign in to your Blogger account at http://www.blogger.com
- Click the Layouts link
- Click the Add a Gadget link
- Copy your search code exactly as it appears in your AdSense account and paste it directly into the 'Content' box.
- Click the Save Changes button
- Click the View Blog link on your Template page