i bought a download game

HI I JUST PURCHASED A DOWNLOAD GAME OF THE SIMS 3 HIDDEN SPRINGS I TRYED TO DOWNLOAD IT TO PLAY AND WHEN I WENT TO REGISTER A ACCOUNT TO DOWNLOAD THE GAME THEY TOLD ME THE SERIAL CODE WAS INVALID AND ALSO THERE IS ABOUT 4 LETTERS AND OR NUMBERS MISSING FROM THIS CODE I WAS GIVEN WHAT DO I DO I NEED THE RIGHT CODE FOR THIS GAME I NEED THE LAST FOUR NUMBERS OR LETTERS TO THE SERIAL CODE THAT WAS GIVEN ME WHERE DO I DFIND THE REST OF THE SERIAL CODE TO THE SIMS 3 HIDDEN SPRINGS GAME