From the common issues thread, which you are supposed to have read.
- I'm having trouble using C&C:Online with Steam.
- Solution: Open the C&C:Online launcher and use the Hook menu to hook your game. Then launch the game using the shortcut Steam made for it on your desktop (if you don't have a shortcut, go into Steam and make one). The launcher should pop up, so select your game and off you go. When Hooking, you should receive a prompt asking for your confirmation, this is how you know it worked. If you don't receive anything and the issue still occurs, check your anti-virus (Norton especially) as they are known to delete the hooking file. Add an exception for C&C:Online.