Well, it's not that easy.
Right, it is not that easy to fathom out what that code is doing exactly because you missed out the bits the define the variables so we can't see if the DAC_CH_0 variable is defined correctly. Bottom line is to post ALL your code.
Also we need to know how you have wired this chip up and what address you are expecting it to be on.
Note that their is an input register, a DAC register and a String register as well as a power down switch, see the diagram at the top of the page. So writing something out is not as simple as writing two bytes of data. You need to write those two bytes and then transfer the input register to the DAC register.
The top four bits of the first byte is the command you want to do the bottom four bits are what you want to do it to. Look at the list of commands to see what your options are.