To check a key assignment,

Press Ctrl + Alt + the plus sign on the numpad. Your cursor will change to a heavy cloverleaf. Immediately press your keyboard shortcut -- in this case, press F11. The Customize dialog box opens and shows you the command the shortcut is assigned to. If nothing happens, there is no assignment. Press Esc to turn off the cloverleaf.

F11 should be assigned to the NextField command. You can assign/reassign keyboard shortcuts in Tools/Customize/Keyboard.