Adding System.Web dll reference to other class libraries

Follow the steps:

1. Project properties
2. Click on the Tab Application
3. In the "Target Framework" drop down, be sure to select ".NET Framework 4" and not the one with the suffix "Client Profile"

for more information:

http://msdn.microsoft.com/en-us/library/cc656912.aspx

The reason you can't find the System.Web is the first sentence:

The .NET Framework 4 Client Profile is a subset of the .NET Framework 4

So, some of the functionality doesn't exists in the client profile, like System.Web.