My private repositories not seen when clicking "Create App"

Hi. I was looking at [the plan description page](https://tabrisjs.com/pricing/) for Tabris.js,
and it says that I can specify one of my private GitHub repositories for cloud builds.
I thought it's a pretty good deal, so I gave a try.
From my dashboard, as I open _"My Apps"_ page, I can see _"Create App"_ button there.
But, when I click the button, I see a list of my public repos alright, but not the private ones...
I tried the synch button several times, but no luck...
I'm sure I am on the _Developer_ plan, which is required for the private repo build.
Is there anything I need to do for my GitHub account? Or, are there any optional settings on dashboard?