Once you work out how to add a widget area in any themes nav menu, you can then simply work out the CSS code to align your Google search (gse) box correctly.
In this post, i’ve already done that for you so you can simply paste the PHP & CSS code in your child themes files.
Screenshot Result – GSE In Twenty Eleven Theme
Here’s what the search box looks like in the primary menu of the Twenty Eleven Default theme for WordPress.
If you want to add the search box in another theme, you can however you will need to find the class for the primary nav menu and tweak the CSS code to align your box perfectly.