LINQPad snippet to read a FreeMind mind map file (.mm) and get the nested Node hierarchy. Also writes formatted lines to file (tab indented hierarchy).

gistfile1.cs

void Main()
{
  var outputPath = "Something.txt";
  var inputPath = "Something.mm";
  var nodes = LoadFromFile(inputPath);
  var lines = nodes.SelectMany(x => GetPrefixedString(x,""));
  lines.Dump();
  File.WriteAllLines(outputPath,lines);
}

static IEnumerable<string> GetPrefixedString(Node node, string prefix){
	prefix += "\t";
	var nodeString = prefix + node.Text;
	return nodeString
		.Unit()
		.Concat(node.Children.SelectMany(x => GetPrefixedString(x,prefix)));
}

static IEnumerable<Node> LoadFromFile(string filePath){
	var xdoc = XDocument.Load(filePath);
	var root = xdoc.Root;
	if(root != null){
		return root.Elements("node").Select(CreateNode);
	}
	return Enumerable.Empty<Node>();
}

static Node CreateNode(XElement element){
	var text = "";
	var textAttribute = element.Attribute("TEXT");
	if(textAttribute != null){
		text = textAttribute.Value;
	}
	return new Node(text){ Children = element.Elements("node").Select(CreateNode) };
}

class Node{
	public string Text{get;set;}
	public IEnumerable<Node> Children {get;set;}
	
	public Node(string text){
		Text = text;
		Children = Enumerable.Empty<Node>();
	}
}

public static class MyExtensions
{
	public static IEnumerable<T> Unit<T>(this T item){
		yield return item;
	}	
}