Ok. Log into your router and look at the existing port forward rule for your PC. This rule should only cover the port 6112 TCP. If it is set for a range of 6112-6119 adjust it to just 6112.
Then load up Warcraft on the PC and go to Options->Gameplay and confirm that the Gameport is set to 6112 to match this rule. Once this is done, it should still be able to host.
On your Mac, you should assign it a static IP address. Go to System Preferences->Network and click the top most connection. On the right it will say this "Ethernet is currently active and has the IP address [xx.xx.xx.xx]". Change the drop down menu from Using DHCP to Using DHCP with manual address. Enter the IP address that is listed at the top [xx.xx.xx.xx] then click apply.
Provided everything worked out, your internet should still function. If not restore your network setting to what they were.
Next up, your Mac will need an almost identical rule to the PC in the router. Log into it and mimic the rule however use port 6113 TCP in place or port 6112 TCP and use your Macs IP address [xx.xx.xx.xx]. Save the change in the router.
Open up Warcraft on the Mac and go to Options->Gameplay and change the Gameport to 6113 to match the rule.
Once all that is done, it should work. If not, go to System Preferences->Security->Firewall and set either Allow all incoming connections, or Set access for specific services and applications. If you opted for the second one, add in Warcraft III.app into the list of allowed programs by clicking the '+'.