Hi Alain,
You might be interested to know that we actually have a French section of the forums where members like yourself can discuss InfoPath in your native tongue - try it out!
As for your question, I think what you are saying is that you can't find the data type to set a reference to the DOM property. If you are using VBScript, which is dynamically typed, you don't really need to do this. An example is below, but I don't do VBScript, so it might be a little off.
Dim myDOM = XDocument.DOM
If you are using managed code you are looking for the IXMLDOMDocument, IXMLDOMDocument2, or IXMLDOMDocument3 data type. I suggest using 2 or 3, see below (in C#):
IXMLDOMDocument3 myDOM = (IXMLDOMDocument3)thisXDocument.DOM;
I hope this helps!
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge