So I selected "data disk" from the options menu and it did add two tracks onto my list, but on the "data disk" screen it is also showing the tracks Great Wall, Red City and Lost Ruin, which I can't seem to get to show up when I'm actually selecting my track. Does anybody possibly know what I'm doing wrong?

Hello. GOG version of motoracer have some problems with tournament mode, and you need to finish it to open all tracks. But there is a solution,if you write the username CDNALSI you´ll open all the tracks.

We´ve talked here this game´s problems.
http://www.gog.com/en/forum/general/about_moto_racer/page1

Thank you for the response. I saw that thread but didn't see anything about the tracks there. Overall it sounds like it's not a problem but actually a feature, which would explain why I hadn't noticed it there :P

No, you didn´t understand me. Of course that´s a feature, but what i mean is that tournament mode can´t be finished because it have many bugs. At fourth circuit the game tells that you have an illegal copy and can´t continue playing!!!! And even if you don´t suffer that bug, some of the stages, especially cross ones and specially Snow Ride, are so unplayable that is really hard to achieve a good position. Even if you can do that it´s totally unfunny.