Mac OS X Mail: Can‘t drag name (vCard) from Address Book to "To:" or "Cc:" message field
You can drag an Address Book name entry (also referred to as a vCard or "contact") to a Mail message "To:" or "Cc:" field by first selecting a different field. This document applies to Mac OS X 10.2.8 and earlier.
Select a different message field before dragging the vCard from Address Book. For example, click the subject field before dragging the vCard to the "To:" or "Cc:" field.