Press the miniature green flag to make the player move. Press the miniature stop button to reset the player. Press the up, left, or right arrow keys to select the block, then click to place it. Press the space key to deselect everything. Press down to delete. When the player touches the activator, it will move in the specified direction. Don't try to jump midair: it doesn't work.
Update! ====== Rearranged levels and bug fixes! ====== Put your level ideas in the comments!