this might seem a bit stupid but I don't know how to add custom parks!
if someone knows (and pretty much everyone knows) it would be great if you helped me.

Put the DAT file in Documents/RCT3/Parks.

well, the parks folder didn't exist on my rollercoaster tycoon 3 folder (BTW im using RCT3 Platinum from GOG) so i created it myself, does that count?

The folder comes with the game no matter what. Its in Documents/RCT3 - There should be a Park folder in there. If not, try and make one.

O_O

embarrassment commence

So you found it then?

yes i did, and im making some fan updates to nerd3's park