It s an old thread, but nowdays you can use .NET. I mean official cross-platform .NET from Microsoft, not Mono. In this case I can recommend my library – DryWetMIDI. Its core part (working with MIDI files, processing MIDI data and so on) is cross-platform for any OS supported by .NET.
As for working with MIDI devices, this API supports Windows and macOS (that s what you need). Full documentation of the library availabale here: https://melanchall.github.io/drywetmidi. There you can find full API reference and help articles. For example, overview article on working with MIDI devices.
The library is distributed via NuGet package and can be installed via NuGet on both Windows and macOS.