To use the weighting network you wire the preamp output to it (through a noiseless R- the value is unimportant- just copy the 6.04k noiseless R8) and specify the measurement node to be the output of the weighting network (V(AWEIGHTOUT)) which was configured to be unity gain at 1kHz. So, this is what you do. Copy R8 (6.04k noiseless) and drop it onto the schematic above the A weighting network. Draw a short line to either end. Copy the AWEIGHTIN pin and attach it to one of the lines. Copy the outmixed1 pin and attach it to the other line. Change the noise simulation output to be v(AWEIGHTOUT). Run the noise simulation and click on the AWEIGHTOUT pin. Measure the noise result as before.