Adding a reference to the assembly in your web configuration file : assembly « Configuration « ASP.NET Tutorial

1. ASP.NET Tutorial
2. Configuration
3. assembly

File: Web.Config

<configuration>
  <system.web>
    <compilation>
      <assemblies>
        <add assembly="MyLibrary,Version=0.0.0.0,Culture=neutral,
          PublicKeyToken=11111111111111111111"/>
      </assemblies>
    </compilation>
  </system.web>
</configuration>