You need to be at OS X 10.6.6 to get the app store. If Software Update isn't working, download the combo 10.68 update from here. Use Disk Utility to repair permissions before applying the combo update, then after installing it, repair permissions again. The combo update provides all update to 10.6, so after applying it, you'll be at 10.6.8 and will have the app store application.
If you previously downloaded 10.7 from the App store, sign in with your apple ID and you should see it on your purchases list.