If you are going to load test a Silverlight web application with WAPT, first of all you need to install the corresponding extension module that you can download here.

This is a very easy task; you should simply download the file and run it. The module enables support for MSBIN1 format used by Silverlight applications to pass data between the client and server parts.

This format is binary. It can represent any complex data structure containing objects of various types, which can, in turn, contain other objects and so on until we come to some elementary objects like text strings or numbers. [...]