Steam/Uniloc confusion. Help needed.

I have bought the game in a shop and brought my laptop to work as I don't have an internet connection at home. When installing I was asked whether I wanted to use Steam or Uniloc. As both are completely unfamilliar to me I went for the default one (Steam). I successfully installed the game and launched it only to realise I will not be able to do the same thing at home as I have no connection. So I reinstalled the game with the Uniloc version but when I enter the same key as I did via Steam I get a message saying it has already been used. How can I un-register my steam thing and use the key for installing the game the "normal" way?