In Xcode even when you highlight text and then bring up the search window, the highlighted text is *not* the search criteria. While I wish there were a way to make Xcode do this automatically, there is a way to quickly get the highlighted text into the search criteria and that is by hitting "Command + E". When you do this, it'll put the highlighted text into the search criteria of the search box. You can do this before *or* after opening the search window.