First, it didn't automatically detect that there was a 'GitHub' remote. My first guess was that I needed to call it 'origin', but that didn't fix it. Instead, I needed to go into the command line and specify the master branch's upstream branch like so: "git branch --set-upstream master GitHub/master".
Second, as soon as I tried to fetch I got "An error was raised by libgit2. Category = Net (Error). This transport isn't implemented. Sorry". Turns out I have to use the 'http' link instead of the 'ssh' link as the remote destination.
Both of these errors could have been avoided automatically, or at least given better help. Branch has no upstream? Assuming that it's the only remote branch with the same name is a pretty good heuristic, especially when you do no work without user action. Don't support SSH? Try the obvious HTTP alternative, or tell/ask the user to try it.