If you have added a reference to log4net correctly and added your usings, you may get a compile error “The type or namespace name ‘log4net’ could not be found (are you missing a using directive or an assembly reference?)”.
This is most likely due to the Target framework being set to .NET Framework X Client Profile. Change this to .NET Framwork 4 (full version) and your application will compile again!
If you want to read more about how to set up Log4Net, read my colleagues blog post Configure log4net from code