How do I install them? How do i find them? Can I use them in multiplayer?

Don't know, don't know and no.
It's not allowed to talk about them here, and this thread will be soon closed.

Buy LFS instead and support its team.

Before you torture yourself by searching somewhere else - the mods just replaces original car's body. You can install the flashiest Lamborghini,it will still have same 140hp as the XRG originally does.
So to repeat the reply above - want better cars,buy a licence. You can start with S1 licence and upgrade later if you're short on money - buying one licence after another will cost the same as buying all together.

There is nothing wrong with the mods.
@racer1012 - you have a PM.

Okay, thanks. I have to start saving up because an S1 license is about 40 fricking RON. S3 is 98. So, do not talk about me getting a license. As i will. also, am i paying 100 ron for just 3 weeks of fun? or is it permanent? thx

No idea what's a RON is,but S3 should be exactly 3 times more than S1 - no more,no less.
And LFS licence is for lifetime.