Have a look to the following example :
# Document creation
[xml]$xmlDoc = New-Object system.Xml.XmlDocument
$xmlDoc.LoadXml("<?xml version=`"1.0`" encoding=`"utf-8`"?><Racine></Racine>")
# Creation of a node and its text
$xmlElt = $xmlDoc.CreateElement("Machine")
$xmlText = $xmlDoc.CreateTextNode("Mach1")
$xmlElt.AppendChild($xmlText)
# Creation of a sub node
$xmlSubElt = $xmlDoc.CreateElement("Adapters")
$xmlSubText = $xmlDoc.CreateTextNode("Network")
$xmlSubElt.AppendChild($xmlSubText)
$xmlElt.AppendChild($xmlSubElt)
# Creation of an attribute in the principal node
$xmlAtt = $xmlDoc.CreateAttribute("IP")
$xmlAtt.Value = "128.200.1.1"
$xmlElt.Attributes.Append($xmlAtt)
# Add the node to the document
$xmlDoc.LastChild.AppendChild($xmlElt);
# Store to a file
$xmlDoc.Save("c:\Temp\Temp\Fic.xml")
Edited
Remark : Using a relative path in Save will not do what you expect.