Sims 3- is there any way i can download from sims store without entering actual serial code when registering?

when installing sims3 i couldn't find the serial code on the guide so i just entered a random one that i found on the internet. Now i want to download stuff from sims store but when registering it asked for my serial number. So im guessing that in order to download it needs to know your serial number. But the thing is i don't remember the one i put in. And its really frustrating because i just found the serial number i should have entered at the back of the guide. So just wondering if the serial code is actually necessary for downloading and whether i can fix the problem somehow. :D