Carolyn,
Thanks for your help. Im downloading the combo update now but when I try to update my mac it says I have the latest version, which it says is 10.5.8
You'll need to do a paid upgrade to Snow Leopard $29 10.6 and then the 10.6 combo update. The App store is only available on 10.6.6! You're still on Leopard 10.5.x.....
You are on Leopard 10.5.x. which is 2 years old now. App Store is made for Snow Leopard 10.6.x. If you have an Intel Mac you can upgrade to Snow Leopard. If you have a PowerMac or G4/G5, you can't. If you want to download third party stuff from one app you can use Bodega http://appbodega.com/index.php