If you are seeing the manual activation screen, it means either your internet connection is not working OR another program such a firewall or a similar security system is blocking online access to FSX, so you must to exclude it or configure it to grant online access to FSX.
However, even the manual activation should work, although it's less easy to use the normal one, and works this way:
- Select the "Register Serial" option and put your Serial Number there
- It will ask if you want to Activate, reply "Yes"
- If there's no internet connection, a dialog will ask you what to do, select "Offline"
- A Window will appear with 3 choices, select "Activate Using a Different Computer that has Web access"
- It will show a code called "Installation ID" related to this machine.
- Go to
http://activate.esellerate.net and type the Installation ID into the field. It should reply back with another long code called "Activation ID".
- Launch FS and wait again for the manual activation screen to come up. Now select "I already have an Activation Key and would like to Activate Now". Type the Activation ID you obtained from the web, and it should activate.