If it's an SPI DAC, why are you not using the SPI pins: D13 = SCK, D12 = MISO, D11 = MOSI, and generally D10 is used for SS as it has to be set to an output on the master SPI device anyway.
Post a link to the MAXIM website for that device.
From this
/* Above: First 3bits(from msb) is control bits last 12 bits is 2867 so expected the output voltage is around 7 volts
*/
I'd say you want this:
digitalWrite (ssPin, LOW); // pin 10
SPI.transfer (highByte (commandPlusData));
SPI.transfer (lowByte (commandPlusData));
digitalWrite (ssPin, HIGH);
and you put the 4 bits + 12 bits together into an int however you'd like.
Perhaps:
int command = 0bxxxx000000000000; // xxxx is command bits from datasheet Table 2
int data = 0b0000xxxxxxxxxxxx; // xxxx xxxx xxxx is desired DAC output.
int commandPlusData = command | data;