Non-CONTENTdm repositories cannot sync OCNs from WorldCat back into their repositories, only CONTENTdm repositories have this feature.
If you are using a CONTENTdm repository and have set up your registration with the Digital Collection Gateway on the Harvest tab, but the sync is not going through, please contact OCLC Support with your symbol, server#, license key and let them know that you are trying to sync OCNs back into your CONTENTdm repository, but the button continues to appear as Register instead of Complete Sync.
There is documentation on how to complete the sync into CONTENTdm.