If you fire up Software Update, you will see that iTunes 7.7 is available. You can find applications in the iTunes Store, but you can’t purchase them unless you have 7.7 installed. The new update is “exclusively for iPhone and iPod Touch” so if you don’t have either one, you don’t need to update.
You may need to go to preferences and check the “Applications” box to make it show up in your sidebar. On one computer it didn’t show up automatically, but did on the other.
The App Store has been a bit finnicky. In the Applications section in iTunes, you can choose “Get More Applications” and go straight to the App Store, but it hasn’t worked all the time. If you search for an app in the iTunes Store, you can get to it, if the App Store link is not working.
As of this writing, software for the iPhone and iPod Touch has not been released, so while you can purchase all the apps you want, you still won’t be able to get them to work on your iPhone. Soon, hopefully, it will be available.