That's relatively simple

Sub SendMessage()
Dim olItem As MailItem
    Set olItem = Application.CreateItem(olMailItem)
    With olItem
        .To = "someone@somewhere.com"
        .Subject = "This is the subject"
        .Attachments.Add LatestFile("C:\path\folder\")
        .Display
    End With
    Set olItem = Nothing
End Sub


Private Function LatestFile(strFolder As String, _
                    Optional strFilespec As String = "*.*") As String

Dim strName As String
Dim strRecent As String
Dim dDate As Date

    Do Until Right(strFolder, 1) = Chr(92)
        strFolder = strFolder & Chr(92)
    Loop
    strName = Dir(strFolder & strFilespec)
    If strName <> "" Then
        strRecent = strName
        dDate = FileDateTime(strFolder & strName)
        Do While strName <> ""
            If FileDateTime(strFolder & strName) > dDate Then
                strRecent = strName
                dDate = FileDateTime(strFolder & strName)
            End If
            strName = Dir
        Loop
    End If
    LatestFile = strFolder & strRecent
End Function