I never did get any help from Red Giant Support & this never worked. In the end I had to insist on a refund. The answer that I got was “Supporting C300 spanned MXF files is on the list for inclusion in a future version, but we don’t have a specific time for when it will be available.”
I see that PluralEyes is now on version 3.5 so maybe that is supposed to support C300 spanned MXF files but as I no longer own a C300 or indeed do video professionally any more I have no idea whether it works or not.