If your equine's passport has no owner details listed, you won't be able to use the Digital Stable look up your equine on the Central Equine Database.
To be able to use the Digital Stable, you'll have to get your passport updated with the owner details by the Passport Issuing Organisation (PIO) that issued your equine's passport.
The PIO's name will be written on the cover of the passport. If your passport was originally issued by a non-UK PIO, it should have been "overstamped" by a UK PIO. This is literally an ink stamp somewhere on the pages of the passport which means that a UK PIO has registered your equine in its records.
If your passport was issued by a non-UK PIO and you can't find an overstamp, then it is likely that your equine hasn't been registered with a UK PIO. In this case, you will need to register your equine with a UK PIO before progressing with Digital Stable.
To find the contact details of your UK PIO, please look here http://help.equineregister.co.uk/equine-passport-issuing-organisations-pios/find-pio-contact-details-using-uelns