Your installing an update and not the program itself.
If you get your OSX install disk, put it into the computer and select option installs from the menu, you will be able to install the iChat program. Then you may follow up with this update.
For Lion / Mountain Lion:
iChat is no longer on these systems. All iChat functionality has been migrated into the new messages app! So just use the messages app. It is installed as default however you can use the preferences menu to add an iChat account!