1. Create a subfolder of App_Code folder named VBCode, and another subfolder named CSCode.

2. Change web.config file as follows:

<compilation debug="false">
    <codeSubDirectories>
        <add directoryName="VBCode" />
        <add directoryName="CSCode" />
    </codeSubDirectories>
</compilation>

Reference:

http://msdn.microsoft.com/en-us/library/t990ks23(vs.80).aspx