[MonoDevelop] problem compiling with monodevelop

[ponto]

i'm starting to use monodevelop and c#.

i was testing xml tools,
and with this code:

// project created on 18-11-2007 at 23:17
using System.Xml;
using System;

namespace xmlteste
{

        class MainClass
        {
                public static void Main(string[] args)
                {
                        //Console.WriteLine("Hello World!");
                        string
url="http://ws.audioscrobbler.com/1.0/user/o_ponto/profile.xml";
                        using (XmlReader reader = XmlReader.Create(url))
                        {

                                while (reader.Read())
                                {
                                    switch (reader.NodeType)
                                    {
                                        case XmlNodeType.Element:
                                            Console.Write("<{0}>", reader.Name);
                                            break;
                                    }
                                }
                        }
                }

        }
}

when i compile using gmcs all works well.

but when using monodevelop with runtime 2.0,
gives me this:
Type=Error, Priority=Normal, Description=The type or namespace name
`System.Xml' could not be found. Are you missing a using directive or
an assembly reference?(CS0246)

how can i solve this?