Add to web.config under configuration/system.web/compilation/assemblies:
<add assembly="my.assembly.blabla,
Version=1.0.0.0, Culture=neutral, PublicKeyToken=373h77ghg78hh478"/>
Like this:
<configuration xmlns="http://schemas.microsoft.com/.NetConfiguration/v2.0">
<system.web>
<compilation debug="true" defaultLanguage="c#" batch="false" numRecompilesBeforeAppRestart="50" targetFramework="4.0">
<assemblies>
<add assembly="my.assembly.blabla,
Version=1.0.0.0, Culture=neutral, PublicKeyToken=373h77ghg78hh478"/>
</assemblies>
...