Google has made some changes to how AdSense publishers can add the Google powered search engine to their sites. You can now find the option to add the search engine to your site in the “by ad unit” tab in the ads section and there is this new button for it.
This button was added at about noon on Thursday, November 12th. Here is the button:
This is that button in the UI, you can see it was added to the other ad units (click to enlarge):
Google said they added it to simplify “the creation flow to make it quicker and easier to create a search engine.” Google also “added a preview, so you can see how ads will look on your search results pages before they go live.”
For instructions on how to add this to your site, see here.
Forum discussion at Twitter.