how do I search for people from AD in a search box provided in SharePoint 2013 Mysite.
That looks like the #SearchBox div. There should be a drop down the that allows you to select conversations, people, everything etc.. I believe you need to configure a search center. It's very easy as it is only a type of Site Collection.
You then have the option to create a new 'Search People`page which does all the heavy query work for you. Here are the steps: