1. When you have said that you have got 0 on the Serial Monitor (SM), I was observing Light upon Light in advance -- ; getting 0 on the SM is the proof that the MAX has accepted its address (0x1D) and has sent back the ACK (acknowledgement signal) to the Master.
2. Button K1 is added to give you some control on deciding the start of conversion of the DAC.
3. Check that the DAC works for some other values. For example: DAC upper 8-bit = 0x3F; DAC lower 8-bit = 0xFF; upload the new program and check that DVM reads about 1.25V. Try, using 0x00 and 0x00; DVM should read about 0V. Also try, using 0xFF and 0xFF; DVM should read about 5V.
Have a nice time!