You need to configure your router to work with your iMac. It may have been done with your eMac in the past, but that is irrelevant now. Head to
http://192.168.1.254 and login to your router. The default login details are
User: root
Pass: 0P3N (zero-P-three-N [in capitals])
If these do not work, I cannot help you further. You can either find the username and password or reset the router back to default settings.
Once logged in, click the Advanced tab at the top of the configuration pages, followed by clicking LAN Clients in the list on the left. The bottom half should show an IP address that matches your Mac static IP address from the Network Preference pane ([xx.xx.xx.xx] that I posted earlier). Select the reserve option next to this entry, then click Apply.
Now over on the left, choose Port Forwarding. Near the top, click on Custom Port Forwarding and enter the following
Connection: Leave this alone
Application: Warcraft3
Source IP: 0.0.0.0
Destination IP: [xx.xx.xx.xx] - as defined earlier
Destination Port Start: 6112
Destination Port Map: 6112
Enable: Ticked
Protocol: TCP
Source Netmask: 0.0.0.0
Destination Netmask: 255.255.255.0
Destination Port End: 6112
Then click apply.
Lastly, click the Save / Restart Menu on the left. Click Save All, and confirm once asked. After it's saved, Restart the router.
After that, provided the port matches in Warcraft, your static IP is the same throughout, and the firewall is off, you should be able to host.
To prevent paranoia, I matched your user name to your account at Whirlpool which had these details.