Pole null is related to the transfer function(y/x) of a system.
When the system is subjected to x input then the y output goes to zero value then its called zero.
similarly if Y is unstable or giving resonance then called Pole.
So take an example h(x)=(x-1)(x-2)/(x-3)(x-4) and just keep the values of x from 1 to 4 you can find that the system behaves zero or unstable.
In other words,
Develop a model which will match to the S21( similar to SPICE etc)
Get the transfer function of this model ( using matlab)
Solve the equations for zero and pole using matlab.
ADS and matlab are integrated so if you work hard it should be possible.