/usr/X11R6/lib/. So if you do not have Apple's X11.app installed, Factor 0.89 will crash on startup. The workaround is to install X11.app.
I applied a fix to darcs and a tester confirmed that Factor worked on their X11-less machine. So this won't be a problem anymore in 0.90. I don't think I'll issue new 0.89 binary packages; this is not a fatal bug, just an annoying glitch.