I'm so glad you asked!
The file you referenced is called 'LynwoodCoaster Sandbox.dat'. When I download it from my web browser, it automatically changes to .ldr (because I installed LDraw, which uses .dat files as well as .ldr files, so the system misinterprets it), so the first step is to make sure that it ends in '.DAT'
Step 2: Locate the 'Parks' folder. From your user, it will be one of the following directories:
• ~/Documents/Atari/RollerCoaster Tycoon 3/Parks/
• ~/Documents/Atari/RollerCoaster Tycoon 3 Premium/Parks/
• ~/Documents/Aspyr/RollerCoaster Tycoon 3 Complete Edition/Parks/
Once you have found it, place 'LynwoodCoaster Sandbox.dat' into it.
Step 3: Launch RCT3, choose Sandbox, select the 'LynwoodCoaster Sandbox.dat' file, and open it. Note that you might need the 'M Brookes' CheatPeepName activated.
Step 4: When you go to save the file, it will start by prompting you to 'Save As…' rather than immediately overwriting the existing file. This will let you preserve the original file as a sandbox for later
Go wild!