You have to create your own vehicle by combining bars, wheels and handles, and then put it in gear, load it and take the green packages to the end... if it moves!
In the edition mode, where you create the vehicle, drag and drop over a wheel to add a wheel to the vehicle. There's also tools of planes and lugs to combine them and build the vehicle: always proceed with drag and drops of the mouse. Once your vehicle is ready, click the button 'generate' to test how it runs; and use the arrow keys to move it. If it doesn't move you should go back to the editor and fix it until it's able to do it. If it works, move it under the unloader and click 'release' so the green nitro packages fall. Once they are loaded in your vehicle, you have to carry them to the end, by moving with the left/right arrow keys and avoiding to lose them on the road.