First, you don't normally need to go to the manual offline activation site, if your PC is connected to the internet. That would be the offline activation procedure, which has been discussed here:
http://www.fsdreamteam.com/forum/index.php/topic,198.msg1031.html#msg1031If your system IS connected to the internet, but the offline activation comes out, it might be caused by one of the following:
- A firewall/antivirus/security program is blocking online access from FSX to the outside.
- Your ISP has a problem with DNS.
The 2nd one seems to be more likely in your case because, activate.esellerate.net WORKS fine here, and it should automatically redirect to one of the several storeXX.esellerate.net sites. If it doesn't, there's a problem with your ISP DNS.
I suggest using Google DNS instead, which are more reliable:
https://developers.google.com/speed/public-dns/docs/using