In order for you to play Left 4 Dead 2 on macOS with minimal graphics settings, your computer must meet the following requirements
Operating System:
MacOS 10.7
Processor (CPU):
Dual core Intel processor, 2GHz or better
Memory (RAM):
2 GB RAM
Graphics Card:
ATI Radeon 2400 or higher / NVIDIA 8600M or higher / Intel HD Graphics 3000
Storage:
13 GB available space