I don’t think you can access the Birthday field as the field object type is reserved. See the following thread for more information about this issue:
In Formativ, you have option to use the native GroupWise address books object or the Formativ wrapping of the address books object. See the comments from the Formativ Language guide:
Read only property which provides access to the Formativ AddressBooks object.
This is a Read Only property. It returns a variable of type Addressbooks.
This object represents the Formativ wrapping of the GroupWise address books. It is provided in order to make it simpler to access fields, especially in multi-lingual GroupWise clients. If you require access to the native GroupWise address books object model, access the GroupWise.Account.AddressBooks property.