Think I'm gonna help you too, because even though Enrique means it well, the English is so bad it confuses people more sometimes.
<LEGACYSTEREOCHANNEL> 1 </LEGACYSTEREOCHANNEL>
to the nebula (reverb or normal) .XML file. Not at the very end, not at the very start. Put it into the middle and you're OK . So this is not on the MAST page, but in the XML file.
Nebula reports itself has having 4 channels input (Stereo in, and another stereo for sidechaining in compressors). Mixbus sees the 4 channels and it doesn't accept it. Making the LEGACYSTEREOCHANNEL xml change makes Nebula report itself as 2 channel input, which Mixbus supports.
Maybe you need to rescan / clear cache-scan or something in Mixbus after making the change.
Thanks, I've already tried that and Mixbus really "accepts" Nebula but only on stereo tracks (although it crashed when opening the plugin GUI). I'd like to have a "mono xml tweak" to be able to use Nebula in mono tracks.