Vertex Fragment Shaders are BROKEN?

Hello, I am trying this shader below from http://en.wikibooks.org/wiki/Cg_Programming/Unity/Billboards

and I keep getting error:

Material doesn't have a texture property '_MainTex'.

Along with these errors:

 Unsupported: Cg  shader for billboards
 Unsupported: Cg  shader for billboards
  done. [Time: 84.483981 ms]
 Unsupported: Cg  shader for billboards
 Shader error in 'Cg  shader for billboards': FindCgParams failed (compiling for flash) at line 7
  
 (Filename: wikibillboard Line: 7)
 
 Shader error in 'Cg  shader for billboards': Vertex program 'vert': unknown input channel 'tex' at line 7
  
 (Filename: wikibillboard Line: 7)
 
 Shader error in 'Cg  shader for billboards': Shader program had errors at line 8
  
 (Filename: wikibillboard Line: 8)
 

Anyone see any problems with this shader? I can't get it working. I'm trying to learn how it works, but hard when doesn't work from the start! :(

 Shader "Cg  shader for billboards" {
    Properties {
       _MainTex ("Texture Image", 2D) = "white" {}
    }
    SubShader {
       Pass {   
          CGPROGRAM
  
          #pragma vertex vert  
          #pragma fragment frag 
  
          // User-specified uniforms            
          uniform sampler2D _MainTex;        
  
          struct vertexInput {
             float4 vertex : POSITION;
             float4 tex : TEXCOORD0;
          };
          struct vertexOutput {
             float4 pos : SV_POSITION;
             float4 tex : TEXCOORD0;
          };
  
          vertexOutput vert(vertexInput input) 
          {
             vertexOutput output;
  
             output.pos = mul(UNITY_MATRIX_P, 
               mul(UNITY_MATRIX_MV, float4(0.0, 0.0, 0.0, 1.0))
               - float4(input.vertex.x, input.vertex.z, 0.0, 0.0));
  
             output.tex = input.tex;
  
             return output;
          }
  
          float4 frag(vertexOutput input) : COLOR
          {
             return tex2D(_MainTex, float2(input.tex.xy));   
          }
  
          ENDCG
       }
    }
 }

EDIT: I tested a sort of similar shader in the docs(the one called "Chess") and get the same type of errors: http://docs.unity3d.com/Documentation/Components/SL-VertexFragmentShaderExamples.html

It appears it doesnt understand texcoord0 in the "Chess" shader, which makes me think it doesnt understand TEXCOORD0 in the wiki shader. So, why is this?...

EDIT2: Did more testing. The problem is definately having to do with texture coordinates used in a struct, or possibly vertex positions. I tried just about all the shaders on the documentation, and none with texture coordinates work. If I use the "ChessOpt" shader, which has no texture coordinate structs, it works great, but the normal Chess shader, does not. I'm starting to think this might be a problem worth bringing to UT attention.