Hi, please make sure you have references to these dlls in your project:
Rebex.Common.dll
Rebex.Networking.dll
Rebex.Terminal.dll
Rebex.Telnet.dll
The Rebex.Net.ProxySocketException is defined in Rebex.Networking.dll and for future uses, there is a log writer in Rebex.Common.dll
In fact, Rebex.Telnet.dll needs all remaining three libraries to work properly, when you deploy your application to production environment.