It's tipically here:
C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\WcfTestClient.exe
Although it will change according to your Visual Studio version (i.e. "10.0") and CPU architecture (i.e. "x86").
C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\WcfTestClient.exe
System.TypeLoadException : Type 'ISomething`1ProxyRandomGuid' from assembly 'DynamicProxyGenAssembly2, Version=0.0.0.0, Culture=neutral, PublicKeyToken=YetAnotherGuid' is attempting to implement an inaccessible interface.
[assembly:InternalsVisibleTo("DynamicProxyGenAssembly2")]