Not sure what the above post means, but in general, you should be running at least 10.6 Snow Leopard (and later if possible as the updates are all free from the app store).

Remove any 'unzip' type applications as you don't need them, 'zipping' and 'unzipping' (aka compression) is built into the OS via an app called Archive Utility (there are certain situations where specialized apps may be useful for compression but not day-to-day).

If you are running 10.5.x at this point, a lot of the newer software installers will give you package errors, which is a result of app certificate problems. The fix is... 10.6 or later.

what should one do if have 10.5 only.

@mariad

i think that computer just became a new paperweight :D

seriously tho, most apps lately only support 10.9 or greater

@DjDCH
great... it works for me..thanks a lot man

@kajitadia

can you please provide more info about your MacOS

Well that would explain why pkg files aren't opening.
You may need to reinstall OS X, recover that file from a backup, or at least track down Installer.app from someone. Installing one of the OS X update packs may fix the issue (assuming they don't require Installer.app to install).
What time of account are you using, as only the root user can modify/delete files in CoreServices (by default)?