I recently installed the Mountain Lion OS on my Mac at work, only out of necessity. I tried to connect my App created in Xcode 5 to my iPhone, and it wouldnt work. I got the error "The identity 'iPhone Developer' doesn't match any valid, non-expired certificate/private key pair in your keychains." I figured out how to create a new certificate, and went to do so. On the way, I realized I needed a Certificate Signing Request (CSR) to do so. Most resources I found on the web pointed to using Keychain Access to create a new one. The link below said exactly what to do
How to obtain Certificate Signing Request
Unfortunately, my Keychain Access does not have the "Certificate Assistant" section that is described in the step-by-step. All that is shown is what is in the image link below
You should see the "Certificate Assistant" section if you click on the "Keychain Access" menu.
It looks like this: