If you haven't found it, the search terms "Register a Sony PRS 600" takes you to the Sony support section for the 600, where it shows an update from November 2013.

This may be of help.

Not quite clear if you succeeded in registering the device , but you would, I hink, also need to register your PC with Reader Store.

Sorry I have no experience of this model, but perhaps another member can help who may still be using one ?