This is by design. BCM does not support customization of its forms.
More BCM answers at http://www.slipstick.com/bcm/bcmfaq.aspx

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Sue Mosher, Outlook MVP
Author of
Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx
> In Outlook 2003, I cannot choose "Design this form" to
> customize the Contacts forms that came with MS Outlook
[quoted text clipped - 7 lines]
> and "Business Contacts" forms that were installed with
> Businesss Contact Manager? Many thanks.