Error 3343: Unrecognized database format 'C:\Temp\Northwind.accdb"

Make sure you have made a reference to the correct DAO library (in VBA, in Tools->References). The old one, for .mdb files only, has DAO in its name. For .accdb files you need the new one, which is called "Microsoft Office 12.0 Access database engine Object Library" (or 14.0, for Office 10).

I got the same error, but it seems the message is misleading. I found this article: http://support.microsoft.com/kb/209953. When I incorporated the DAO.Workspace object and the password in the OpenDatabase arguments from that article into the code you posted, it worked.

Sub UsingDAOWithWord()
 Dim docNew As Document
 Dim ws As DAO.Workspace
 Dim dbNorthwind As DAO.Database
 Dim rdShippers As Recordset
 Dim intRecords As Integer
 
 Set ws = DBEngine.Workspaces(0)
 Set docNew = Documents.Add
 Set dbNorthwind = ws.OpenDatabase("C:\Temp\Northwind.accdb", _
    False, False, "MS Access;PWD=northwind")
 Set rdShippers = dbNorthwind.OpenRecordset(Name:="Shippers")
 For intRecords = 0 To rdShippers.RecordCount - 1
 docNew.Content.InsertAfter Text:=rdShippers.Fields(1).Value
 rdShippers.MoveNext
 docNew.Content.InsertParagraphAfter
 Next intRecords
 rdShippers.Close
 dbNorthwind.Close
End Sub

I'm a Word guy, not so much an Access user, so I'm not sure what made OpenDatabase throw that particular error, or how you're supposed to find out about these requirements.