Hi, thanks for posting!
It definitely looks to me like NCrunch is not aware of the postsharp build step binaries and is therefore not copying them into its workspace. You should be able to confirm this by right clicking on the failed build in the Tests Window, choosing Advanced->Browse to workspace and checking to see if the workspace contains all the postsharp files needed to build.
The 'Additional Files To Include' configuration setting would definitely be the thing to resolve such a problem. Make sure you add a full reference to your PostSharp tools relative to your project file (unless you're specifying the config at solution level, in which case this is your solution file). You may even want to take a step further and include ALL the packages for your solution in this configuration setting. At solution level, this would probably be something like:
packages\**.*
And at project level, it would probably be:
..\packages\**.*