Right from the toolbar, you may search all your files at once via the global search field in the top right corner of the Texpad toolbar (swiftly reached by pressing Shift-Cmd-F).
The find function is supplemented by an optional replace once the results have been displayed. The search is displayed in the same pane as the output. To go back to the PDF or the Error View, choose the option in the toolbar or simply press Cmd-8 or Cmd-9 respectively.