I'm guessing you want to download the light pack, and the steelworx supports pack?
Both download links are in my FAQ(first post in this thread) under 'Custom Scenery.'
You want Pumper steelworx(for the custom supports), and DRP's Lightz.
Click on the links in there, and they will give you instructions on how to download, and what to do.
All you have to do is burn the files you download to a CD, then install it on the RCT3 compy.
Theirs tutorials on installing custom scenery in my FAQ as well.