Hm... I haven't had enter not work yet to start it, but enter on the keypad and space should work as well. On gamepad, A/Bottom Facebutton/X(On PS controller) should all work as well, and also the start button on the controller. If you are in the WebGL version in might take a few seconds to load, and for all versions you may need to click into the game to make sure it has focus.