Open Data Source Data Merge

On pag 304 of Word 2004 AppleScript Reference I have found this script:

tell application “Microsoft Word”
set myDoc to make new document
set main document type of data merge of myDoc to document type form letters
open data source data merge of myDoc name “Macintosh HD:Development:DataSource.doc”
end tell

If I run it I got an exception:

AppleScript Error
Microsoft Word got an error: data merge of document “Document3” doesn’t understand the open data source message.

Any ideas ?