Debugging on an iPhone device

I have a developer license and I've generated .mobileprovision and .cer files. I've put the provision file on my device, double-clicked the .cer file and added it to the login keychain. But when I'm trying to build the app in XCode, I get the error: CodeSign error: code signing is required for product type 'Application' in SDK 'Device - iPhone OS 3.1开发者_如何学Python.3'

What am I doing wrong? Thanks in advance!

Is your private key associated with the certificate in the Keychain utility?