I had that problem when I purchased the same world bundle pack from walmart a while back. Only on mine, after figuring out how to fix several errors Hidden Springs finally installed and it said Monte Vista code was already used.
Log out and close your game and launcher, then log into your thesims3.com account, - then go to the "Store" tab - Scroll down to "My Store Account" - Then click on "Purchase History"
It should show up there to download to your game launcher. Once it is downloaded to your sims 3 game launcher then open up "Downloads" section -, Hidden Springs shows up under your lots tab. make sure it shows under the status column as installed. if it is not installed just check the box and click install. Then you should be ready to play in your new Hidden Springs town. (Monte Vista shows up under the objects tab - so while your in your launcher make sure its downloaded and installed to your launcher/game also) Its better just to do 1 download and/or install to the game at one time. I have never had luck checking multiple things to download and install at the same time
if it works let me know - If not and you get a error message let me know that too
Only Monte Vista is under my history. Still says code use.
on thesims3.com under "My Page" does only Monte Vista just show up under the worlds you own or is Hidden Springs showing as owned too?
My worlds bundle pack had me install Hidden Springs 1st and then Monte Vista and they both had a different activation code, So make sure you are not using the Monte Vista Code
(Edit - Update: One of my other problems was my game/launcher was not up to date- if you use origin/windows/pc it should be Version 1.69) - but I was getting script errors and other things along with the code already used problem)
if you after you have tried:
1. Logging out of your launcher, game, and the sims3 website and closing out all of them
2. Delete the IE browsing history (even if you do not use IE - the game does and there is always stuff to delete)
3. Open up your launcher and log back in then check under the downloads to see if it showed up (Hidden Springs shows up under Lots - Monte Vista under Objects)
If it still is not on your launcher in your downloads then
4. Log back into your sims3 online account and check your My page again and the Purchase history to see if it showed up to download to your launcher
if that does not work
You might have to contact EA customer support either using the live chat or call them on the phone (the phone option will usually give you the option to have them call you back so you do not have to wait on hold for the next available person - as long as its during their business/call in hours) :
https://help.ea.com/en/contact-us/
I do not work for EA, but I was hoping since I had the exact same problem that what fixed mine would work for you too
Let me know if any of that works or what you find out or If you still have problems
