ErrorAfter selecting the network for processing I don't see the FID or OBJECTID fields in the Polyline ID drop-down menu in the layer to process panel?

SolutionThese fields have been deliberately exclude from the list as they can cause some tools used by RivEX to function in unexpected ways. You need to have another field which is a unique numeric field for your river network. RivEX can build you a field called RivID for you, more information about that is found here.