Does anyone know how to download this mod and where you put it?

Do you mean the Relativity mod?
You just download it like any normal mod and put it in your "Mods" folder. (If you're unsure how to do this, perhaps you should read Sims 3 CC Basics under "Help" on this forum.)
You'll need to set it up in game though - I suggest reading through the FAQs and Interactions subposts (in the NRaas page) to find out how to do this to your liking.

Just a tip for in future - it's best to ask NRaas questions in the NRaas forums, simply because that's where the downloads are housed and they have appropriate forums over there for questions such as these.
Happy simming!