Well, I guess this is one way to have a first post here...

I had to register here after digging through pages worth of Google results on one issue. Recently, I downloaded a wooden coaster design for RCT3 (the stock wooden coaster, not a mod), but can't seem to find a way of actually getting it into the game.

I put the "[WoodenRC]name.trk" file for this coaster design in Documents/RCT3/Coasters (I think that's what it was called), but got into the game, and dug through every little button trying to figure out how to LOAD the coaster, before I finally quit and hit the 'net. One popular YT tutorial I found (https://www.youtube.com/watch?v=G9aOQ1YGHoQ) said to use the "import designs from previous RCTs" function in-game. Well, I did that, and even let it search my entire hard drive to find the file, but it still didn't work. Some other people have said there's some button for it in the coaster building interface, but all I see there are the buttons for construction.

Seems like I'm outta luck here, but I hope someone here knows a way to fix this mess. If it has any relevance, I play the GOG version of RCT3 Platinum.

First, welcome to the site! I certainly hope you'll enjoy your stay and visit the forums on occasion.

As for the problem you're faced with, have you tried clicking specifically on the folder icon in the coaster interface?

It looks specifically like this: 

   

When looking at the Coaster Selection Menu it appears like this:

   

If you have been using that I in no way mean to try and make you sound stupid, a lot of people miss that button thanks to Frontier's 'fantastic' GUI design. From the way I interpreted what you've typed though I think this should solve your issue (hopefully).

I hope this helps!

Oh jeez, do I feel like an idiot. That worked perfectly. My only problem now is finding some space in my park for the thing. xD

The sandbox park is usually kind of small. I'm thinking of releasing a bigger version of it