Island Paradise Installer Wont Open on Mac

I downloaded the Sims 3 Island Paradise and have been trying to install it all day. This is the first time i've done this so i'm not really sure on much but i trawled the forum trying to find answers.
My computer is a mac, and i found the mac guide and followed all the steps, but when it came time to click on the installer app it just popped up on my dock and bounced there, with nothing happening, and then when i click it, it just goes away.
help me, i don't know what else to try!!

Admin edit: Moved.

- ADMIN

Hello,

Have you created .dmg disk file? Please read this Mac guide for creating.dmg files and installing process.

Yeah, i have and i did everything it said and the installer still wouldn't open.
i deleted and re-downloaded in case something had happened to the file as i was downloading it and am trying again now.
i think the problem is none of my files will open with stuffit. i know i have the latest version but every file i download, from this site or others, it is saying that the file was made with a newer version of stuffit or that that archive is incomplete.

i just finished making the dmg and it still wont open. it's doing the same thing as before

First off did you get this installed correctly since the 1st?

If not did you get StuffitExpander from the stuffit website or the Mac App Store?