What is a zener diode's I_z rating?
Your question refers specifically to CircuitLab. Here is the dialog in question: 



It is clear here that what they call I_z is actually Izt. They call the dynamic impedance of the Zener R_S, on the datasheet it will likely be called Zzt, and it should be the value at Izt. 

The datasheet will probably give you a maximum value for Zzt, which is likely much higher than the 'typical'. SPICE models usually use the typical values. In the particular case of the 1N4742, the datasheet maximum is 9 ohms, which is almost an order of magnitude higher than the typical.