Just seen it on Mixbus forum but it may not work on version 1.3.600....
The Windows version of Mixbus is unable to load VST plug-ins with more than 2-in / 2-out I/O. Acustica Audio's Nebula 3 plug-in includes 4 inputs by default, but here's a way to work around this restriction:
Note: If you have already attempted to load Nebula 3 in Mixbus, you will need to clear the plug-in cache and re-scan after implementing these changes.
1. Navigate to your default plug-in folder and open the folder labeled "acustica."