Xcode Problem

Hi there. I'm relatively new to programming for the iPhone, and I have Xcode 3.1. As a starter, I decided to download videos from Apple to help me learn how to use Xcode for the iPhone better. So I did what they said: create a new project, make it a Cocoa Touch List application, and name it "Sample". Then click "Build & Go", and you SHOULD have the iPhone simulator open up with a list of countries and whatnot. Doesn't work for me: it gives me 1 error and 1 warning. It says, "gcc-4.0: installation problem, cannot exec '/Developer/usr/bin/arm-apple-darwin9-gcc-4.0.1': No such file or directory" and "CodeSign warning: code signing is not enabled for product type Application Wrapper". In the tutorial video, it showed the man being able to run his application on the simulator with no problem.

Also, when I try to "build & go" with a blank iPhone OS Application (such as a Tab Bar Application), it builds correctly, but then says, "Error from Debugger: Failed to launch simulated application: iPhone Simulator failed to install the application." What's going on here? This never happened with applications for Mac OS X, and this happens even when I have something in the application, when it's not blank.

Thanks for reading this, and if you have anything that might be able to help me out, I'd be glad to hear it. Thank you very much.