The structure of the DAQmx API documentation makes this a bit confusing. The page you link to (NI USB-6002 Supported Properties) indicates that the "Trigger Type" property is valid for the 6002 (because it is), and it takes you to the page for the "Trigger Type" property. This latter page is not distinguished by device, so it lists all possible values for the property across all DAQmx devices, not just the 6002.
The USB-6002 is not capable of analog triggering, just digital (on PFI0 and PFI1). This information is available in the NI USB-6002 Specifications.