one question, is the ID number my serial number?
That's your problem: you are confusing the installation id with the Serial Number. In the "Register Serial" dialog you should enter your Serial Number only, and the Addon Manager will even prompt you the starting letters ( for example, "FSDTKLAX-" ), so it's obvious what you should enter here.
Now, if you have correctly turned off ALL your security software (antivirus AND firewall), there should be nothing else to do, and the product will activate online automatically.
If the program still suggest to do a web-based activation, because you were unable/unwilling to turn off all your security software, so the activation server couldn't be reached from FSX, you can do a web-based activation.
Use the "Register Serial" dialog and insert your
Serial Number. If the activation server cannot be contacted (for the above reasons, caused by firewall/antivirus), the program will prompt to do an offline activation. Reply YES.
In the next screen, select this option:
1. Activate using a web browser on this computerWhen you are prompted to click on the activate.esellerate.net link, the
installation id is already pre-filled. Don't touch it,
just press the Submit button. After you pressed the Submit button, the activation site will give you a code, the
Activation Key. Go back in the sim, and select this option:
3. I already have an Activation Key and would like to activate nowInsert the
Activation Key, as requested.