We had to add a new feature to our charts to enable the dashboard inclusion. As a result, charts saved prior to the release of the dashboards need to be "resaved". You can use the "open" feature in the dialog (the "folder") to navigate to your chart to resave. Once you've saved the chart, it will be selectable in the "load" dialog.
Can I load charts saved locally (JSON) to the dashboard?
Not yet, but we are working on it.