Terrain textures

Since Unity didn't come with some basic terrain textures, I've attempted to 'borrow' some from other unity projects, just to get started. The problem I have now, is Unity doesn't see them, what file do I add these textures to, so that I can begin using them? I still think it would have been easier to just include some basics, but since that didn't happen, a new user like myself is left to fumble through all of this. I really think this program has great potential, but if you are new to it, you've really got a steep learning curve just to do the basics.