We just did a SWLab episode yesterday that talks about this very setup, including diagrams and Cisco commands.
Solarwinds Lab #16 Service Pack 1: Configuring Span Ports, Packet Analysis and Netflow Correlation
In short, the 'node' for the Network Sensor, should be the node hosting the Network Sensor, which should be connected to the port of the switch where you are mirroring the SPAN traffic.