Before assuming the problem is with the application try this. Navigate to Users/YourName/Library/Preferences/
com.apple.installer.plist and delete that file. Now try opening a .pkg?
Still not? Create a new account, name it "test" and see how your Installer.app works in that User acct? (That will tell if your problem is systemwide or limited to your User acct.) This account is just for test, do nothing further with it.
Open System Preferences >> Accounts >> "+" make it an admin account.
Let us know and we'll troubleshoot this further.
-mj
macjack@gmail.com