The key signature tool can be found in the 4th pane from the right in the palette. It is the icon of the 3 sharps. Simply tap the icon, choose your key signature, and tap the "Insert" button. Make sure that the green status bar is in the location where you wish to add the key signature, prior to using the tool. If you take a look at page 33 of the User Guide, you can see a map of the "Instrument Score Techniques 2" that illustrates where the key signature button is. The User Guide can be accessed by tapping the icon of the "?" on the Home screen of the app.