you have to start nebula once in your daw, to have it generated. then you will find it in "Library/Audio/Presets/AcusticaAudio/Nebula3". there are two library-folders in your imac. one is directly on the mac-harddrive, the other is in your user-directory. here on my mac i find nebula in the main-library directly on the mac-harddrive.
hope this helps, mathias
system 1: windows 8 32 bit - samplitude prox, tracktion6, reaper system 2: mac osx yosemite - reaper(32+64bit), tracktion6(32+64bit)
both systems on: macbook pro (late 2009), core 2 duo 3,06 ghz, 4 gb ram, graphic: nvidia geforce 9600M GT 512 MB