How do I sync/merge my local Xcode git repository with a remote git repository? I have an Xcode project with a local git repository. How do I set it up so that I can sync/merge my local repository with a remote repository that I have already created?

I've tried going to the Organizer and adding a remote repository, but I got a certificate error when trying to add GitHub.
You need to connect your local git repository to the remote git repository. Unfortunately, Xcode (as of v4.6) doesn't provide a UI to do this. Since you are using Github for your remote repository, I would advise that you follow Github's instructions for adding a remote.