Prior versions of Mixbus ( v1 and v2 ) provided a utility called "JACK" which connected Mixbus to your soundcard; it also provided the ability to interconnect multiple applications. For example, you can use JACK to connect the outputs of ProTools to Mixbus. ( here's a video showing an old version of Mixbus, and an old version of ProTools: https://www.youtube.com/watch?v=QxGhN8w5D1s )
We no longer encourage people to "route" audio between workstations because it doubles your CPU load, and it adds complications with save/recall, timecode sync, and latency. Instead we encourage users to "export" their tracks as wave-files, and import those files to Mixbus. That's the same process you would use if you were taking the tracks to an analog studio for mixing
On Windows and MacOS, JACK has become (mostly) a tool for developers and is no longer widely used. JACK is no longer necessary, or desired, for Mixbus version 3 and newer.
Mac users: Jack for OSX El Capitan (and newer)
The version of JACK that was included in the Mixbus v2.5 package was released long before El Capitan. The JACK installer has been updated to work with El Capitan. This is a BETA package, but several users have tested it: