Hello Hardlight,
I will agree with you, you have picked the best of the three RCT games! Even though I have, for the most part, retired from building RCT2 designs--though I will continue to upload old ones--I always appreciate it when others create and upload their RCT2 creations. The RCT2 community here is nowhere near as big as the RCT3 community, but there are still several of us who can be seen uploading and even commenting in the RCT2 downloads section. I myself have some halfway decent RCT2 creations on my page, if you're interested.
Downloading is the easiest thing you can do on this site; all it takes is the click of a button, and you don't even need to be logged in. Every track, park, scenario or other download will have a big blue "Download" button next to it. When you find that button, all you have to do is click it, and the track you downloaded should appear in the game as a premade track.
Uploading has a few more steps behind it, but it isn't difficult once you learn the tricks behind it. First off, before you upload anything, make sure you've taken screenshots of it. Screenshots are required for every upload. If it's a track you're uploading, I suggest you also take a screenshot of the statistics panel (excitement, intensity, nausea...) so you won't have to memorize it for later.
There's a big blue button that says "Upload Your Works" that can be found in many places around the site. If you ever have trouble finding it, it can always be seen in the "Downloads" section, which can be accessed from the top of the page, among other places. Once you've clicked that big blue button, it will take you to a pretty straightforward page that asks you to fill out the name and file type of the download. That's the easiest step - just give your download a name and tell us if it's a track, a park, or one of the other options given.
Step 2 is the part I had the most trouble with, since finding your own saved designs isn't quite as intuitive as finding the default designs that come with the game. My computer runs Windows 7, so I access my custom designs by following these steps:
Local Disk -> Program Files (x86) -> Infogrames Interactive -> Roller Coaster Tycoon 2
Then, after you're in that folder, find and click the button that says "Compatibility Files" in the toolbar. You should see all of your screenshots, if you have any, and a several folders. If you want to upload a track, click on the folder that says "Tracks" and you should see all of the tracks you've saved onto RCT2. Select the track you wish to upload, then proceed to Step 3, which is another easy and straightforward step that you'll be able to figure out once you get there. This is the step in which you're required to state the excitement rating, intensity, etc. for the track your uploading, and this is where that screenshot I advised you to take earlier comes into play. If you're uploading something other than a track, such as a park or a scenario, just type in a description of whatever it is you're uploading.
Step 4 is where you upload the screenshots of your content. I already mentioned how you find those, so as long as you've already taken them, you should be able to find them. You can always save screenshots somewhere else, too, so it's easier to find them. I'm fairly certain that you have to change the file type of the screenshot in order to upload it here, so you'll want to do that in advance and save them in a more accessible place, such as a folder on the desktop. I always open Paint and paste my screenshots there, then re-save them as jpeg images.
Step 5 is just there for you to make sure all of the information you've provided is correct. You can always go back and fix any errors in your upload before you publicize it.
Anything else you would like to know about uploading or about RCT2, I'll be here at least for the next several Built It! rounds.