IEnumerable Elements 'Where' and 'Select' In Unity?

I have a C# program that has a function that reads an XML. I took that function and brought it over to my Unity game which will be using the same XML. However, my reading method doesn't seem to work.

Libraries:

 using System.Xml;
 using System.Xml.Linq;

Reader:

 static public XDocument ProjectXML = XDocument.Load(SaveHandler.gFolder + "xml");
     List<XElement> GameInfo = ProjectXML.Descendants("Project").Elements("Subsystems").Elements("Values").Where(x => x.Attribute("Name").Value == "GameInfo").Elements("Value").ToList();
     foreach (XElement elem in GameInfo)
     {
         if (elem.Attribute("Name").Value == "TerrainGenerationMode")
             GenerationMode = (elem.Attribute("Value").Value == "Flat");
         if (elem.Attribute("Name").Value == "WorldName")
             WorldName = elem.Attribute("Value").Value;
         if (elem.Attribute("Name").Value == "TerrainLevel")
             TerrainLevel = int.Parse(elem.Attribute("Value").Value);
         if (elem.Attribute("Name").Value == "TerrainBlockIndex")
             TerrainBlock = int.Parse(elem.Attribute("Value").Value);
         if (elem.Attribute("Name").Value == "TerrainOceanBlockIndex")
             OceanBlock = int.Parse(elem.Attribute("Value").Value);
     }
     PlayerPosition = new double[3]; // Temporary Line
     List<XElement> Player = ProjectXML.Descendants("Project").Elements("Entities").Elements("Entity").Where(x => x.Attribute("Name").Value == "Player").Elements("Values").Where(x => x.Attribute("Name").Value == "Body").Elements("Value").ToList();
     foreach (XElement elem in Player)
     {
         if (elem.Attribute("Name").Value == "Position")
             PlayerPosition = elem.Attribute("Value").Value.Split(',').Select(double.Parse).ToArray();
     }

That code is perfectly fine in my other program. But apparently Where and Select don't exist in Unity's libraries.

So what are my alternatives in Unity?

EDIT, Errors:

 Error    1    'System.Collections.Generic.IEnumerable<System.Xml.Linq.XElement>' does not contain a definition for 'Where' and no extension method 'Where' accepting a first argument of type 'System.Collections.Generic.IEnumerable<System.Xml.Linq.XElement>' could be found (are you missing a using directive or an assembly reference?)    F:\Repositories\SC TB 3D\Assets\Survivalcraft\Settings.cs    22    111    Assembly-CSharp
 Error    2    'System.Collections.Generic.IEnumerable<System.Xml.Linq.XElement>' does not contain a definition for 'Where' and no extension method 'Where' accepting a first argument of type 'System.Collections.Generic.IEnumerable<System.Xml.Linq.XElement>' could be found (are you missing a using directive or an assembly reference?)    F:\Repositories\SC TB 3D\Assets\Survivalcraft\Settings.cs    37    107    Assembly-CSharp
 Error    3    'System.Array' does not contain a definition for 'Select' and no extension method 'Select' accepting a first argument of type 'System.Array' could be found (are you missing a using directive or an assembly reference?)    F:\Repositories\SC TB 3D\Assets\Survivalcraft\Settings.cs    41    75    Assembly-CSharp