Hudson : warning MSB3245: Could not locate the assembly "nunit.framework"

I'm working in a Virtual Machine as a Hudson Slave. In the virtual machine, I add the path to the MSBuild I want to use :

%SystemRoot%\Microsoft.NET\Framework\v4.0.30319\;

I added also the path to NUnit :

C:\Program Files\NUnit 2.5.7\bin\net-2.0;

And so, I can run the NUnit and MSBuild command lines in my VM.

But I get this error when my Hudson Job build my solution :

C:\WINDOWS\Microsoft.NET\Framework\v3.5\Microsoft.Common.targets : warning MSB3245: Could not resolve this reference. Could not locate the assembly "nunit.framework".

What should I fix in my VM or in Hudson? Thank you a lot!

EDIT

Here how the concerned file *.csproj references to the dll :

<Reference Include="nunit.framework, Version=2.5.7.10213, Culture=neutral, PublicKeyToken=KKKKKKKKKK processorArchitecture=KKK">
    <SpecificVersion>False</SpecificVersion>
    <HintPath>Dll\nunit.framework.dll</HintPath>
</Reference>