NUnit: Could not load assembly nunit.framework

Trying to execute a unit test which references nunit.framework fails:

System.IO.FileNotFoundException: Could not load file or assembly
'nunit.framework, Version=2.5.10.11092, Culture=neutral,
PublicKeyToken=96d09a1eb7f44a77' or one of its dependencies. The
system cannot find the file specified.

My scenario is the following:
Visual Studio 2010 TFS
C# and .NET Framewok v4.0
NUnit 2.6.0.12051

Detailed explanation:
I have a test project which references some dlls, one is nunit.framework.dll. This dll (nunit.framework.dll), points to the folder where nunit 2.6.0.12051 is installed. I execute NUnit (not from visual studio), through nunit-x86.exe and once lauched I load the test project(it is a class library, dll). Nunit tries to load but the error above explained appears.Test project compiles Ok, the dll is generated correctly. Also test project is targeting to .NET Framework v4.0.