In Java I use "import", in C# I use "using", in VB6 what I should use?

For example, I write a method:

Private Sub Command1_Click()
  Dim dom As New DOMDocument
  Dim http As New XMLHTTP
  Dim strRet As String
  If Not dom.Load("c:\voucher.xml") Then MsgBox "文件不存在"
  http.Open "Post", "http://172.31.132.173/u8eai/import.asp", True '指定服务器ASP
  http.send dom.xml '把xml数据发送服务器端
  strRet = http.responseText '
End Sub

I need to import "DOMDocument","XMLHTTP" and so on, what should I do?

In the IDE select menu item Project > References and then check "Microsoft XML, v6.0" (I think)