Assuming you want basic differential and not mixed-mode S-parameters, then you can attach a "balun" component (from the "rfLib" library) to both the differential input and output of your amplifer. One would then attach a "port" component (from the "analogLib") library to single-ended terminals of the balun. The port should configured with the correct impedance (e.g 50Ohms) and assigned a port number (1 and 2). Depending on your design you may also need to add DC blocking and biasing to the amplifier inputs, etc.
The "rfLib " library is available in the cadence installation directory: $CDS_INSTALL_DIR/tools/dfII/samples/artist/rfLib
You should now be ready to simulate the design in spectre using ADE. Setup an "sp" analysis and sweep over the desired frequencies. After the simulation completes, select "Results->Direct Plot->Main Form..." You can also use the calculator to plot S12.
You may also want to check the RF Design Forum for addition help and suggestions.