Not sure what to tell you since the only option I see is to reinstall the game yet again.
Something probably went wrong with the installation and seeing how many people have launcher issues after reinstalling the game after that stupid DLC error message which isn't even necessary since it's a bug, this is a major issue that EA needs to fix asap.
So, please don't bite my head of when I say: Clean your computer from the game and reinstall from your disc. To get the expansion use the solution posted in the link above.
Try uninstalling following the instructions here: http://simswiki.info/wiki.php?title=Game_Help:TS3_Uninstall
