To measure the i/O impedance you have to:
1- Connect an HB 1 tone port at RF with the number 1
2- Connect a termination (the one used for S parameters) at IF with the number 2
3- Connect a current generator in parallel with a 50 ohm resistor at he LO port and express the current of the generator as dbmtoa(PLO,50) where PLO is the LO power.
4- Set the LSSP simulation with 2 tones RF and LO and set the frequencies at the ports as: port 1 RF, port2 RF-LO (or what is your IF)
5- After the LSSP parameters are caluculated use the function Zin_RF=zin(S(1,1),50) to get the imput impedence at port 1 or S(2,2) for port 2.