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 support@aspyr.com for further technical support.