The version of OSX doesn't matter (as long as it's 10.7 and above). We prefer Mavericks since that's what most people probably have.
To be a tester, you must be able and willing to do the following:
- Accept our non-disclosure agreement
- Download a .zip file, extract it, and install an .rbz plugin into SketchUp (8, 2013, 2014 (whichever you have))
- Locate the install location for plugins in the chosen versions of SketchUp and delete installed plugins (like Twilight Render Version 1, and Version 2)
- Delete (at least temporarily) Twilight Render Version 1, if necessary (there may be conflicts between V1 and V2)
- Explore and use V2 as much as possible with limited direction
- Post clear descriptions of problems and results, with screen shots