It is very simple to attach the wheels to the axle or fork.
Rear wheel: press the button in the center of the wheel, then slide the wheel onto the axle and release the button. Pull the wheel slightly towards you until you hear a click. The rear wheel is now attached.
Front wheel: Press the button on the top of the mounting block. Slide the front wheel with the pin into the mounting block. Release the button and the front wheel is attached.