Input impedance is quite simple to be evaluated. U may have 2 cases:
1) a 1-port model: in this case the input impedance equals the Z11;
2) an N-port model (N>=2) and when ports are strongly coupled: in this case the input impedance at the port 1 is Zin=(1+S11)/(1-S11)*re(Z0_port1). You can define this within the output variables dialog box when u do 'create report'-->'rectangular report'. Similarly for other ports. Obviously the S11 parameter must be insterted 'dimensionless'.
Hope this helps.
Ivan