You need to go on their official website and register I think. https://www2.citiesxl.com/index.php?option=com_comprofiler&task=registers&Itemid=62
The activation key should have provided by steam, or otherwise can be found easily (I cant remember exactly where but its in the steam FAQs (easy to find)).
Once you register just follow what the site says, tbh never owned the game, and from what steam forums say, i dont think i ever will, im sorry to say it, but it doesnt seem too great (especially with the memory leak problems).
This particuliar problem is not steam related (i think) so steam is not likely going to refund you. But if you feel like you want to try to, then contact steam support by submitting a ticket.