I've always heard that there isn't one. If you really want to make levels, you'll need to either buy a PC and get a copy of JCS for Windows, or find a PC emulator for your Macintosh.

Nothing really personaly against Macs as computers, but they're generally not a good platform choice for gaming, simply on the basis that there's so little support compared to PC. You'll have to expect this kind of general lack of support. I was amazed that Epic even made a Macintosh port of the game to begin with.

Anyway, good luck.