This works for me (dates are in dd/mm/yyyy format - yours are probably different):
set BirthDate to "29/5/1979"
set AneeDate to "12/01/2001"
set AneeDate to date AneeDate
tell application "Address Book"
make new person with properties {first name:"John", last name:"Smith", birth date:date BirthDate}
tell person "John Smith"
make new custom date at end of custom dates with properties {label:"anniversary", value:AneeDate}
end tell
save addressbook
end tell