Run through the Adafruit tutorial for the board. You should be able to produce a triangle wave and sine wave at the DAC board VOUT pin. This ensures you have connected all the wires and soldered the pins correctly.
Read the datasheet and read the library source code to understand how it works.
I2C is a communications bus to the DAC chip.
Look at the source code for the triangle wave example.
dac.setVoltage(counter, false);
The setVoltage function sends a message over the I2C bus to the DAC chip. The DAC chip then changes the voltage output based on the first parameter. Stop thinking in duty cycle and start thinking in voltage output. The program changes the output voltage from min to max then max back to min. Scope the DAC board VOUT pin. If you do not know how to use a scope, you are in over your head.