MailMerge.MainDocumentType (Word)

Returns or sets the mail merge main document type. Possible return values are wdCatalog - Catalog, wdDirectory - Directory, wdEMail - Email message, wdEnvelopes - Envelope, wdFax - Fax, wdFormLetters - Form letter, wdMailingLabels - Mailing label, wdNotAMergeDocument - Not a merge document.

Set myDoc = Documents.Add 
myDoc.MailMerge.MainDocumentType = wdCatalog