Can someone PLEASE tell me how to SAVE a building I've created in a park AND open one I've downloaded?? PLEASE!

In the scenery menu, there should be a button that looks similar to a wooden building - like in this screenshot:

http://www.rctgo.com/images/soaked/soaked_40.jpg

It's the second from the bottom in that screenshot - I believe there are a few more after it in Wild, but that's just to show you the icon.

Now, once you click on that, a third menu is brought up with something like a save icon, an open icon, and a paint icon. Clicking on the save one will let you select land, which will then include the building pieces inside that land - I'm not sure if it includes other scenery, but I think it does. There is then some button you press to save it, but I don't quite remember right now.

To open one, just click on the "open" icon, and it'll list the buildings inside your My Documents / RCT3 / Structures folder. From there, you select one, and it'll let you place it wherever you want.

Hopefully that works for you! I haven't messed around with that menu in a while so sorry if I am wrong on anything, but I believe that's how it works.