The Dash uses a HID profile for programming, so when in program mode (double-blink on System LED) you will see a HID device but not a serial port.
You should not need a driver on Win10 as Microsoft has finally figured that out.
Make sure your sketch calls Serial.begin(); in setup() if you intend to use the USB serial port.
Windows OS handles port number assignment. Resetting the Dash or taking it in or out of program mode can cause Windows to re-assign the port number. If using the Arduino IDE terminal window, you may need to close the terminal window and check that the correct port is selected.