The sims 3 pets expansion trouble

hello,
Ive downloaded the sims 3 pets, and used stuffit to expand it. Im getting an icon that says the sims 3 pets installer but when I try to install it, it comes up with an error message saying "Could not verify the integrity of the installer".
I have tried the steps showen in the fixes part of the forum but you really will have to excuse me, I am in no way good with computers, and could not find the files to "Unpack" or anything.
Any help would be greatly appreciated thank you
Katie

ey that tut is outdated.

you have to make a .dmg and copy past just all in it.
not the sims 3 pets.app open than folders no
just all folders with the .app in the .dmg the mounted file.

sorry for late repy.