Also, you might wanna try to delete (backup first!) your profile located in /Users/<Username>/Documents/SimBin/Race07 - maybe something's broken in there.
Another usual culprit would be the visual settings - navigate to R07's folder (<steam directory>/steamapps/common/race07) and launch the Config.exe file with administrative rights. Set up everything, save, and try starting the game again.
While this might seem far-fetched, I've seen those settings break R07 with something as simple as a graphic card driver update