I found that I had to unplug my joystick, then my mouse worked in game.
With the joystick plugged in I was getting suggestions to use A' or 'B' buttons in the menus for moving back & forth, which was a bit of a hint that the game thought I was using a gamepad, but with the joystick unplugged the 'A' and 'B' controller icons went away and mouse control returned.