Supposedly, a value of 1.0 represents full sun, straight up, on a cloudless day. You could reduce the default value to represent less than perfect weather conditions, or a tilted panel. If you find a way to use mirrors to increase the illumination of the panel, you could increase this number, but probably not much.
Generally pretty close to the car's actual mass. Units are kg. If you have very heavy wheels, you should add a term representing their rotational inertia as well. The model won't let you put in less than 0.14 kg, because we measured the mass of a panel and a motor as 0.100 kg and 0.037 kg, respectively.
Multiply the actual radius of the wheel by your gear ratio. Units are meters. So for a 4 cm diameter wheel, an 8 tooth gear on the motor, and a 32 tooth gear on the axle, enter 0.02 m * 8 / 32 = 0.005.
The following assumptions are based on Chuck Wright's analysis of his kit's solar panel and motor. I have yet to verify these for my own. If you want control over these parameters of your model (because you measured different values, not because you will cheat and modify the components provided), you need to download the program and change the values yourself. I don't provide a web interface to change them.
Of course, you can make your car slower than these simulations, but I don't think you'll make them any faster. While this software can give you guidance on what's important and how to adjust a car for speed, once you have a car it's important to measure and adjust it for real.
And of course, there's more to the contest than speed. Other categories include "Best Engineered" and "Most Creative", or something like that. And please, don't forget that computer simulations should never substitute for real-life measurements and honest understanding.