Not sure what you mean here.  there are 2 possibilities, though.
(My apologies, these examples are in C#, but the fundamentals are the
same - Also, they have not been sanity checked, purely because it is
nearly time to go home.  ;o) )

1)  Dataset.WriteXml("filename.txt",XmlFileMode)

or

2)
string output = "<?xml version=\"1.0\"">;
foreach(DataRow myRow in Dataset.Tables[0].Rows){
 output += "<row>";
 foreach(DataColumn myColumn in DataSet.Tables[0].Columns){
   output += "<" + myColumn.ColumnName + ">" +  _
myRow[myColumn].ToString() + "</" + myColumn.ColumnName + ">";
 }
 output += "</Row>";
}

then save "output" to the file.  

HTH