Sims 3 Pets Could Not Verify Integrity of Installer

So I've read all the forums on here relating to this issue & tried the disk image thing but it didn't work for me. I downloaded it from here:   
I think I already have a disk image but when I click the installer, it says "Could not verify integrity of installer."
Its supposed to work for Mac according to some video I've watched. I've downloaded & installed Generations, Island Paradise, University Life, & Seasons just fine but Pets won't seem to work ): Please help, I don't wanna redownload it again that took forever.

Hello,

Unfortunately, it seems that you are not using downloads from games4theworld. Because of that, I am unable to render you assistance since I do not know how the files you have downloaded are supposed to work. I suggest that you seek help from the uploader of the file?

I will only be able to assist you if we are talking about the same files, if you understand what I mean  I'm sorry about that and I wish you good luck!

where can I download sims 3 pets from games4theworld for mac then?

Here you go: https://games4theworld.forumotion.com/t53-all-games4theworld-downloads



HOW TO INSTALL THE SIMS 3 ON A MAC (simplified)


(Please click HERE for detailed instructions!)


* If you have previously installed The Sims 3 on your mac, please do a FULL and CLEAN uninstall before proceeding.

1) Install your base game.
2) *Update your base game to the latest version via the launcher's Game Updates tab. [Current: V1.63]
3) Unzip original Expansion Pack files with Stuffit Expander. (Skip Steps 3 & 4 if you have downloaded your EPs in .iso or .dmg)
4) Create .dmg files of the EPs with Disk Utility.
5) Install EPs in order of release.
6) Apply the LATEST Ultimate Fix for Mac. (Use ONLY the two files in the main #Crack folder!)
7) Enjoy your game!

Again, if you are unsure about one of the steps, please view THIS GUIDE for images and detailed instructions!

* If the update fails via the launcher for some reason, please download the manual patch instead HERE.