i own it and want to download it

Do you have it on disc or did you buy the game from Origin?

If the latter just download the Origin client and log in with the same email and password that you use on the Origin store. Here the download link for the Origin client:

http://www.origin.com/download

If you have the disc version you have to register the game on the Sims website first: 

https://www.thesims3.com/registeragame.html

After that it will show up in your Origin client and you can download it from there. Please make sure that you use the same email and password for both the Sims website and the Origin client.