You should do this in CEX mode.
Here is an extract of the XMB CFW Settings Readme by mysis:
=BD Remarry=
1) Toggle Factory Service Mode (should be easy now in Network->cfw-settings->Toggle Factory Service Mode)
2) Put "eid_root_key" that belongs to the console to /dev_usb/
3) Select Remarry Bluray Drive in Network->cfw-settings->Remarry Bluray Drive
4) If everything went fine it should have notified: "CEX_drive_init() succeeded"
5) Get out of FSM
6) Dont forget to repair CRL/DRL if you need to fix it to play BD videos, try the multiman method first!
Successfully remarried /dev_usb/cfw-settings.log should look like: