If you are trying to track what keywords visitors to your site are using to search your site or the web in Google Search box, you can use a little workaround (hack). Enable the custom logo feature and optionally adjust the color scheme of the AdSense for Search results.

Web access log file will list the referral string of what page called the custom logo image. A simple script probably can fetch the list of keyword out of the log file.

Google now enables AdSense for Search publishers to view top queries report in AdSense control panel.

Wondering what your users are searching for on your site? There’s now an AdSense for search {Top Queries} report that will let you do just that. This new report shows you what your users are looking for, by listing the 25 most common searches conducted through your AdSense for search boxes. Use this report to identify additional topics to add to your site, or to keep track of your most sought-after information.

From What are AdSense for search top queries:

AdSense for search top queries are the top 25 queries performed on your AdSense for search box within a given date range. Please note that the top queries report does not show unique queries; only queries that receive two or more hits are displayed.

So if you want more than the top 25 keywords in the report, old method still works.

Source: JenSense.