If you are getting this error:
Could not load file or assembly 'System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.
First check that you have Microsoft.Web.Extensions.dll and Microsoft.Web.Extensions.Design.dll in your bin folder. Also check that you are using the correct version of .net in IIS.
If non of these solutions work then verify that if application is in a folder other than root you make this folder an application in IIS. Right click folder "convert to application".