@Nelson > A serial number is only required when you are using the PERFORMANCE mode of rekordbox; the EXPORT mode will always be available for free.
The XDJ-RX does not come with a serial number as none is required to use the EXPORT mode. The RX is currently not supported in the PERFORMANCE mode, should that change in future you would need to purchase a license (a free 30 day trial is available as you have discovered).
If you are unable to export to a USB device, it could be that it is not recognized by rekordbox due to the file format. What is it you're trying to export to?