You want to locate the ‘debug.keystore’ on your machine (should be in the Corona folder) and use that for (Android) builds till you’re ready to do production builds (at that time you’ll create your own signing cert for that specific app).
I’d also uninstall Corona, reboot, and re-install. That might fix the issue you’re having with the blank field.
By default, Corona should select the ‘debug.keystore’, but both OS X and Windows simulators use their respective OS’s file history mechanisms. So, if for some reason the simulator is looking in the wrong place for the keystore, it will continue to do so till you ‘point it back in the right direction’. i.e. Locate the ‘debug.keystore’ and select it in the build dialog.