OK, while the steps required have been mentioned by others in this thread, let me just outline the steps required on both machines.
This post assumes that the following IP addresses will be manually assigned:
Mac: 192.168.0.1
PC: 192.168.0.2
To the best of my knowledge the base IP address itself (192.168.0) does not matter but I'm just using a common convention here.
On the Mac, do the following:
1. Open System Preferences > Network
2. Select "Built-In Ethernet" from the list of network adapters and click "Configure"
3. Select "Manually" from the "Configure IPv4" menu on the TCP/IP tab
4. Set "IP Address" to 192.168.0.1
5. Set "Subnet Mask" to 255.255.255.0
6. Set "Router" to 192.168.0.2
7. Select "Apple Now" and you are finished on the Mac
On the PC, do the following:
1. Select Start > Settings > Network Connections
2. Right-click on your Ethernet connection and select "Properties" from the menu that appears
3. In the General tab of the Local Area Network Connection Properties window that appears, scroll down the list of connections until you find "Internet Protocol (TCP/IP)" - select it and click the "Properties" button
4. In the General tab of the window that appears, select "Use the following IP address" radio button
5. Set "IP Address" to 192.168.0.2
6. Set "Subnet Mask" to 255.255.255.0
7. Set "Default Gateway" to 192.168.0.1
8. Select "OK" to close the window, then "OK" again in the remaining open window, and then you are done on the PC
What you've basically done here is assigned an address manually to each computer and told it to route its outgoing traffic to the other computer by specifying the other computer's IP address in the Router/Default Gateway field. The computers should now be able to talk to each other directly and, in theory, you should be able play CoD2.
Hope that all makes sense and I hope it works out for you.