gcc-4.2 failed with xcode 1

Question

I have started learning opengl es. I downloaded code from the companion website, but I cannot compile any project. When I use simulator 4.0 as base SDK, I get the message

/Developer/Platform/iPhoneSimulator/Platform/Developer/user/bin/gcc-4.2 failed with exit code 1.

However, whet I use device 4.0 as base SDK, I am getting the message

Code sign error:The identity iPhone Developer does not match any certificate/private key pair in the default key chain.

This happens in all the opengl projects I downloaded. How can I compile this projects?