I can't see anything wrong with that code at the moment; more information is needed. Try renaming the class name (ItemName) to the name of the item's json file.
It say
Building mod Notch
Validating Jsons ...
Compiling code ...
Apocalypsio.cs (7.32)
Type 'Microsoft.Xna.Framework.Vector2 is defined in an assembly that is not referenced. You must add a reference to assembly 'Microsoft.Xna.Framework, Version = 4.0.0.0, Culture = neutral, PublicKeyToken = 842cf8be1de50553'.
npcID NPC.NewNPC int = ((int) p.Center.X - 300, (int) p.Center.Y - 300 NPCDef.byName ["Notch: EyeOfApocalypse"] type 0.)
^
Apocalypsio.cs (44.32)
'Microsoft.Xna.Framework.Vector2' does not contain a definition for 'X' and no extension method 'X' accepting a first argument of type 'Microsoft.Xna.Framework.Vector2' was found (a directive using an assembly reference or is it missing?)
npcID NPC.NewNPC int = ((int) p.Center.X - 300, (int) p.Center.Y - 300 NPCDef.byName ["Notch: EyeOfApocalypse"] type 0.)
^
Apocalypsio.cs (67,32)
'Microsoft.Xna.Framework.Vector2' does not contain a definition for 'Y' and no extension method 'Y' accepting a first argument of type 'Microsoft.Xna.Framework.Vector2' was found (a directive using an assembly reference or is it missing?)
npcID NPC.NewNPC int = ((int) p.Center.X - 300, (int) p.Center.Y - 300 NPCDef.byName ["Notch: EyeOfApocalypse"] type 0.)
^
Apocalypsio.cs (38.38)
'Microsoft.Xna.Framework.Vector2' does not contain a definition for 'X' and no extension method 'X' accepting a first argument of type 'Microsoft.Xna.Framework.Vector2' was found (a directive using an assembly reference or is it missing?)
Main.PlaySound (15, (int) p.Center.X, (int) p.Center.Y, 0);
^
Apocalypsio.cs (55.38)
'Microsoft.Xna.Framework.Vector2' does not contain a definition for 'Y' and no extension method 'Y' accepting a first argument of type 'Microsoft.Xna.Framework.Vector2' was found (a directive using an assembly reference or is it missing?)
Main.PlaySound (15, (int) p.Center.X, (int) p.Center.Y, 0);
^
Apocalypsio.cs (5.38)
Type 'Microsoft.Xna.Framework.Audio.SoundEffectInstance is defined in an assembly that is not referenced. You must add a reference to assembly 'Microsoft.Xna.Framework, Version = 4.0.0.0, Culture = neutral, PublicKeyToken = 842cf8be1de50553'.
Main.PlaySound (15, (int) p.Center.X, (int) p.Center.Y, 0);
^
Apocalypsio.cs (5.38)
Type 'Microsoft.Xna.Framework.Audio.SoundEffect is defined in an assembly that is not referenced. You must add a reference to assembly 'Microsoft.Xna.Framework, Version = 4.0.0.0, Culture = neutral, PublicKeyToken = 842cf8be1de50553'.
Main.PlaySound (15, (int) p.Center.X, (int) p.Center.Y, 0);
^
Apocalypsio.cs (5.39)
The type or namespace name 'Vector2' not found (a using directive or an assembly reference is it missing?)
Vector2 npcPos = p.position - new Vector2 (0.200);
^
Apocalypsio.cs (39,39)
The type or namespace name 'Vector2' not found (a using directive or an assembly reference is it missing?)
Vector2 npcPos = p.position - new Vector2 (0.200);
^
Failed to build Notch.