First of all, you need to find your RCT2 directory (since TD6 is the extension for an RCT2 track file). For most people it's C:\Program Files\Infogrames Interactive\RollerCoaster Tycoon 2. Once there, go in the "Tracks" folder. Now you'll need to know the name of the track you're uploading. Find that, and you're done!
I hope that helps.
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