Custom entities and relationships to system entity

Hi

I have created a new entity called Category and I've created a 1:N relationship with the system entity Account...basically I am wanting to categorise my Account records.

I have added the relationship field to the Account form and it looks correct in that it is a lookup field.

The problem I have is when I open the Account form and navigate to the new field it is appearing as read-only although this has not been set for this field.

I don't know if this is something to do with security roles but no matter what I select it does not seem to "open" the field so I can select something from the list.