On UNIX based machines it is not possible to bind applications not running from root to any port below 1024. So the workaround is to to redirect port 80 to any other port say 8080 and then forward this port (8080) to port 80 of virtual machine using virtual machine settings. Now you will b able to access web server from your browser using the address
Here is how you can forward port 80 to port 8080 in Mac OS X
sudo ipfw add 100 fwd 127.0.0.1, 8080 tcp from any to any 80 in
Figure 1 shows port forwarding from host machine to guest machine in Virtual Box.