The DGX-230 doesn't have MIDI ports; it has a USB port. You don't need a USB-MIDI adapter for it; you just connect the DGX-230 to a computer with a USB cable.
If you're trying to connect another MIDI device to the DGX-230, you can't do that directly via a USB-MIDI adapter. You'll need to use your laptop as a go-between so the data coming from or going to the DGX-230 can pass through Yamaha's USB-MIDI driver:
(1) Connect the DGX-230 to your laptop via USB, and make sure the correct Yamaha USB-MIDI driver has been installed on the laptop.
(2) Connect the other MIDI device to your laptop via the USB-MIDI adapter, and make sure the correct USB-MIDI driver for the adapter has been installed if necessary.
(3) Use a DAW to connect to the MIDI ports and to route the data as desired, or else use a MIDI router utility such as MIDI-OX.