• Go to Tools -> Options -> Debugger -> General
• Uncheck the option Enable Just My Code (Managed Only)
• Uncheck the option Enable. NET Framework source stepping (this is optional)
• Check the option Enable source server support
• Uncheck the option Require source files to exactly match the original version

symble servers:

• http://referencesource.microsoft.com/symbols
• http://srv.symbolsource.org/pdb/Public
• http://srv.symbolsource.org/pdb/MyGet
• http://msdl.microsoft.com/download/symbols

to debug unity source code

http://unity.codeplex.com/releases/view/31277

download symbols:

http://unity.codeplex.com/downloads/get/467070

reference:

http://thechefprogrammer.blogspot.com/2013/04/debugging-into-aspnet-mvc-4-source-code.html

http://*.com/questions/13609891/how-to-step-into-mvc4-source-code-without-building-assembly