The mean free path is, as the name suggests, the average distance a molecule can go before colliding with another molecule. Logically, we can expect this to be mean speed of the molecules (c) multiplied by the time between collisions (1/z where z is the collision frequency)

l = c/z

and if we substitute with the equation for z on the bottom of the collision frequency page, we have

l = kT/ppd²Ö2