Because the NI 5620 is a Traditional NI-DAQ device, it will not appear under the list of associated digitizers when you right-click on your device under NI-DAQmx Devices in MAX. MAX will have a section for Traditional NI-DAQ Devices, the 5620 will appear in that list.
The proper way to configure the NI 5600 downconverter and the NI 5620 high speed digitizer as an NI-5660 RF signal analyzer is to initialize the individual devices together programmatically within LabVIEW using the NI 5660 VIs. The NI 5660 VIs can be found in LabVIEW within the Functions palette under Instrument I/O»Instrument Drivers»ni5660.
The ni5660 Initialize.vi function has an input for downconverter device number and digitizer resource name. The downconverter device number specifies the number that traditional NI-DAQ assigns to the NI 5600 downconverter in MAX. The digitizer resource name specifies the number that traditional NI-DAQ assigns to the NI 5620 digitizer in the format DAQ::DeviceNumber. Refer to the following screenshot and the NI-RFSA shipping examples specific to the NI 5660 (such as ni5660 Demodulate AM VI) for more information on how to program the device using the ni5660 Initialize.vi
Refer to the Getting Started Guide linked below for more information about the configuration of this product.