For those who seem to be following the answers above but still have the problem (e.g., see the first comment on the poster's question):
You are probably working in a solution with many projects. The project you appear to be working in references other projects, but you are actually modifying a file from one of the other projects. For example:
System.Web.Extensions
But if the file you are modifying to use System.Web.Script.Serialization
is in project B, then you will need to add a reference to System.Web.Extension
in project B as well.