1) If you have an internet connection you can unlock the game by typing in your username and password. This works fine for like 95% of the world.
2) If that doesn't work for some inexplicable reason, you can switch to the "serial number" tab and enter your serial number. This probably covers an additional 3% of the world.
3) If THAT doesn't work (likely because, like you, you're on a PC which has no Internet connection), you can click the "Manual Unlock" tab, and follow those instructions. They are admittedly a bit sucky, but they
do work, if you follow the instructions. You can save or print the manual unlock request code, and we'll happily validate it as soon as humanly possible (usually extremely quickly) and get you on your way. It's a bit of an inconvenience for that small percent who (like you) sadly haven't got Internet on their gaming PC, but it does work. Again, sorry that you have to jump through a hoop. Your PC would be happier if it had the internet, you know.