Also look under your edit menu for "Preferences," or hit Command/CTRL-K (depending on your system). In the main preferences dialog box, up at the top is the setting for your Redo hotkey. By default, Photoshop is set to toggle Redo/Undo with Command/CTRL-Z. Setting Redo to Cammand/CTRL+Shift-Z will allow you to use the Command/CTRL-Z for multiple Undos.