Looks like NCrunch doesn't know about ExtensionSDKs and doesn't copy those to its workspace.
Here is some info:
http://msdn.microsoft.com/en-us/library/vstudio/hh768146.aspx#ExtensionSDKs
I ran into this because I used this for my project:
http://visualstudiogallery.msdn.microsoft.com/772ae9fd-b926-46c1-bc2d-2ffbde159c2b
Which installs an ExtensionSDK. In my project file, it looks like this:
<SDKReferenceDirectoryRoot>$(ProjectDir)\libs;$(SDKReferenceDirectoryRoot)</SDKReferenceDirectoryRoot>
</PropertyGroup>
...
<SDKReference Include="XUnit, Version=2.0">
<Name>XUnit for Managed Projects</Name>
</SDKReference>
I can workaround by using "Additional Files to Include", of course, but I think NCrunch should copy these automatically.
Leonid