If your game did not come with a serial, contact EA for support.
If it did and does not work,
1) Verify that you are typing it in correctly. Remember that O and 0 are not the same key!
2) Try installing using a different optical drive. Sometimes the drive cannot read the disk to verify the serial.
3) If rundll32 is running in the background, this may cause issues when attempting to enter the serial.
If all else fails, contact EA for support.
Click here for EA's serial replacement instructions.