50cj5

Okay i really need help i bought arma 2 CO and everytime i try to run DayZ it says bad serial number given everyone says to just run as administartor and run arma 2 and arma 2 arrowhead but they wont launch i have done both of the cache file things multiple times and i have turned off my computer and restarted but i think i need to run arma 2 before i can play Dayz but as i said it just wont launch plz help me!!!