I was talking to madmatt (i get the same bug), he said it was probably my VRAM. I have the same imac with ati rage 128, CMAK requires a 16 meg card and mine is only 8 (but it was the same for cmbb, which runs fine).
The thing that puzzles me is that CMBB ran fine with no black thingos, and most of the CMAK terrain is the same resolution as CMBB's, especially stuff that should give me trouble (eg grass, tree bases, buildings)
Here is a screenshot:
I am thinking it is probably my VRAM also, I heard someone say there is no more downsampling?
Also, I tried switching to a lower res grass, that didn't do it. Going to try a few more things.