That's exacly what I did. I tried both altitude and distance renders hoping for better results, and neither one works (above is the altitude mask). It renders the ball parts as flat gray, and there should be a gradient. Maybe it's a bug in the Mac version. Or maybe it's a bug with the metaballs. Or maybe it's a different bug altogether, I don't know. I've noticed that metaballs don't always mask properly. I've tried it with the metaballs grouped and ungrouped (what I've found to be the bug with masking metaballs) and still, I get the same results.