How to use both NUnit extension and the NUnit VisualStudio Test Adapter

I have some problem to use both the NUnit Visual Studio Test Adapter and a framework extension class I've created. In particular my solution has 2 projects: the first one is a class library that contains some methods I must test and the second one is the test assembly. This assembly includes the extension class (I don't add the related dll to the addin folder inside the NUnit program directory because that extension class has been created exclusively for this assembly) that I have created to get and use the assertion failure messages.

Question: I would like to have a BIN folder (next to my solution folder) where place all the DLLs I need, and use this BIN folder to run my tests from Visual Studio (that is why I need the NUnit Visual Studio Test Adapter) and through the NUnit-x86.exe program (I mean through the NUnit GUI). At the moment I can run correctly all my test only by running them from Visual Studio... through the NUnit GUI the extension class does not work (I mean that my extension is invisible, I don't mean that there is something that gets error).

How do I have to set my project?

Extra info: I'm using NUnit 2.6.4 and VS 2013 Professional on a 64bit machine. To use the test adapter I followed this guide: http://ift.tt/1oooCOl