Using extension methods in .NET 2.0?

I want to do this, but getting this error:

Error 1 Cannot define a new extension method because the compiler required type 'System.Runtime.CompilerServices.ExtensionAttribute' cannot be found. Are you missing a reference to System.Core.dll? [snipped some path stuff]

I have seen some answers here that says, you have to define this attribute yourself.

How do I do that?

EDIT: This is what I have:

[AttributeUsage ( AttributeTargets.Assembly | AttributeTargets.Class | AttributeTargets.Method )]
public sealed class ExtensionAttribute : Attribute
{
    public static int MeasureDisplayStringWidth ( this Graphics graphics, string text )
    {

    }
}