Setup port forwarding:
1) Find out the IP of your PC or MAC where MAMP is running - let's assume this is 192.168.1.100 2) Go to your router and find port forwarding - setup forwarding form external -> 192.168.100 for port 80
3) Save the configuration and test
here is a screenshot that may help
In your case the ports below would be STArt = 80 and End = 80