Hi JJ,
I fixed the problem with grass generation on OSX 10.10.x There was a mistake in the shader code causing a duplicate declaration that evidently didn’t cause problems on PC or previous versions of OSX. Unfortunately my fix didn’t make it into Preview 2 but it will be in Preview 3 and the main 4.8 release. But it’s easy enough to apply manually.
The fix is to replace your /Users/Shared/UnrealEngine/4.8/Engine/Shaders/LandscapeGrassWeight.usf file with the one in this zip file.
Shaders should recompile when you load your level, but you may need to remove the invalid grass by running the ‘grass.flushcache’ console command in the editor. (~ key)