Hi,
DCIn is where you can add a switch if you want a switch on/off button. Or a jumper.
If you want to power from USB you need USB5V and USBGND.
Remove USB5V only if you need midi data but with 9V DCIn supply.
Makes sens ?
I know it's confusing, but 5V, GND and 3V3 GND are not Jumper. Just pins where you can get this voltage.
It works on my macbook, the device is called "DFU in FS Mode".