Hi, thank you for trying out the game!
In regard to the controls, the jump button is indeed the down arrow key, this is because the mapping is D-PAD = WASD and Action Buttons being Arrow Keys, with start and select being Q and E(which doesn't have any function at the moment in this game). This is stated in the Controls section of the page. Obviously depending on feedback, will determine whether I control with keep this type of control scheme that I'm testing out(as it's one I've personally used over the years), but I apologise if this has lead to confusion.