Question: I enter the serial number for Call of Duty 2 and it returns with a message that the serial number is invalid. What can I do?
Answer: If you get this error, this means that the your Internet connection cannot reach the registration server. This happens if you have a firewall or your ISP is blocking the connection to the Digital River registration server.
Please try the following:
1) Go to System Preferences and click on Security & Privacy. Then click on Firewall. Make sure the firewall is turned off.
2) Directly connect your Mac to the Internet (for example, don't use your wireless connection).
3) Try a different Internet connection.
If the above still fail, please contact the developer of the game at email@example.com for further technical support.