Sorry if this is in the wrong section, but I really want to know where my sandbox parks are stored so I can upload my parks.

Help or a guide or something would be appreciated. ^^

You should be able to find them in the following directory:

My Documents\RCT3\Parks\

How do you on a mac???????

How do you on a mac???????

I found this on youtube and I think it may help you...
Rct3: How to find you "Parks" folder (MAC USERS)
https://www.youtube.com/watch?v=69QHuDxE2GM

also.. (by L-33) (please look at the full forum thread, before trying)
(I am not responsible for anything you may try regarding the links I provided for you.
http://www.shyguysworld.com/index.php?topic=14677.0
"If anyone else is having this same problem, the parks get saved into Library/Containers/com.aspyr.rct3.appstore/Data/Library/ApplicationSupport/RollerCoasterTycoon3Platinum/Parks."
I hope I helped or gave you a place to start.