How do i get access to SOAP response property?

Try using this way

var xmlResponse =xmlhttp.responseXML.documentElement;

var fullNodeList = xmlResponse.getElementsByTagName("loginUserSoapOutPart");

for (var i=0; i < fullNodeList.length; i++)
{
  var eachnode = new Option();
  eachnode.text = fullNodeList[i].childNodes[0].nodeValue;
  eachnode.value = fullNodeList[i].attributes[0].value;
}