DllNotFoundException on Mac OS X, only when Playing in Editor

I'm using a custom DLL that has a dependency to another DLL. On Windows, I was able to make this work by using the function from @bibbinator here, storing both the DLL I'm using and the dependency in the same folder within Plugins. When I open the project on Mac and try to Play it in the editor, I get a DllNotFoundException error; the same error I was receiving on Windows before the bibbinator fix.

When I try to build the project, however, it builds perfectly fine, without any errors.

Thoughts as to why this might be happening?