To see what startup isos have been added are in columns for the top of the startup disk. To open the iso left click on the iso and press the Open button.
If you want to find an iso image if it is not shown press the Other button and a file dialog to open the location will show up. The process of creating a startup disk to boot off deletes all data on the disk but only shows USB devices if you hard USB hard drives or flash drives with data you want to keep it is recommended to unplug these devices so there is no way of accidentally deleting data you care about.
