Why can I install mods?!?!!

So firstly I have a problem as where I have Eletronic Arts in my Documents but not Sims! Only Sims Castaway Stories! (I got that) Then I started searching I went on computer os c and then users - My name and Documents Eletronic Arts and Sims 3? Is it the same thing?

Yes, those two folders are the same!
But if you have a "The Sims 3" folder in one, it should also be in the other...

But I can't find the Package files Folder?

Oh, it's not there by default. You need to create it.
Download this, and extract it. Then move the Mods folder to Documents\Electronic Arts\The Sims 3
Inside the Mods folder, you'll find a Packages folder. That's where you'll put the .package files.
By the way, the two files that are there are to remove the intro, and to remove the stars animation when building something. If you want to keep either of them, feel free to delete the file!