Hi Sam -
NCrunch emulates the deployment behaviour of MSTest, which basically involves setting up a separate sandbox for each test while that test is executed. This can be a source of confusion, as NCrunch also builds its own workspace (so we're basically talking about a sandbox within a workspace). The behaviour of MSTest within these sandboxes is configuration-specific and varies depending upon the version of MSTest used, which makes it very difficult to standardise across test runners.
Try adding an a 'DeploymentItem' attribute to your test, specifying the path to the .zip file. This will make NCrunch's MSTest adapter aware of the file so that it is copied into the MSTest sandbox.
Another option is to use the location of the test project binary when trying to find the file. For example, you can make a call to Assembly.GetExecutingAssembly().Location and use this information to infer the location of the file, assuming it is positioned relative to the test assembly.
I hope this helps!
Cheers,
Remco