Help d2 installation?

HELP ME PLZ!!!!!! i screwed up so bad my mom is going to kill me so i have to write this fast so here it go's i found d2 cd key for expansion on a website i uninstalled d2 to put the d2 expansion cd key in but it says that it is from a different product is there any other way and how do i make it work becuz if i cant my mom is going to slaughter me lol not really butr plz help me fix my dilima