i bought S2 about year ago and play it at laptop, than im not playing about half a year and want to continue playing at PC, help please! how to unlock S2 at PC?

Go to the unlock screen and enter your LFS account information (login and password) and it will unlock S2 on your pc.
You will have a maximum of 2 unlocks on your account and will get a new one every friday, in case the maximum of 2 isn't reached yet.

im trying but the game write 'demo-racer'

Then you haven't entered the correct login and password.
You need to enter the name and password you used when you bought the game.

Maybe it is because you have a demo account... (unlees you are not posting from the account you are trying to use).