Because WD sent you a new unit the system config file import will be of no use for restoring users or shares. From page 66 of the My Cloud User Manual:
Before doing a factory restore or a system update, you may choose to save your device’s current configuration. At a later time, you can import a previously saved configuration. Keep in mind that importing a configuration after restoring factory defaults does not restore shares or users. See “About Safepoints” on page 71.
In the User Manual on page 76 through page 78 it details how to perform a SafePoint Restore. The manual makes this important point:
Make sure the replacement My Cloud device is new or is in factory fresh state. (See “System Factory Restore” on page 64.) The capacity of the replacement device must be the same or larger than the safepoint you created.
Further the manual appears to indicate that one can restore only from a USB drive attached to the My Cloud or from a location on the local network. If one has moved or copied their My Cloud Safepoint backup offsite one may have to copy or move that Safepoint back to the local network or USB hard drive in order for the My Cloud to see, access, and restore the previous Safepoint backup.
If one used a 3rd party backup program or service to backup their My Cloud one shouldn’t expect the Safepoint program within the My Cloud to open that 3rd party backup file set (or access that 3rd party service) to restore the files via the Safepoint interface. Instead one may have to use the 3rd party program or service to initiate the restore process to the My Cloud.