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