Can't get script to find native OS X plugin in Unity 5

I've got a native C++ OS X plugin generated by X Code and copied over.

The bundle is called rtmidi-wrapper.bundle and I've put it in my project's Assets/Plugins folder. (I know in Unity 5 is doesn't matter any more where they go.) In the Editor, the bundle shows up as a plugin in Inspector with the various options, with "Any Platform" selected.

In a C# script, I have the usual [DllImport( "rtmidi-wrapper" )] directive before external function signature. This is giving me the dreaded DllNotFoundException: rtmidi-wrapper message.

Everything looks like it should work according to the documentation. (And I have some Android plugins that work fine, with the "lib" prefix on the library file name.)

What have I got wrong? If nothing obviously wrong, how to debug this?