They suggest black coffee.  However, certainly on day 4 you can add milk. 

Also, if you are doing the new CSD there is a dairy allotment each day. 

There are a couple of arguments against coffee, however, I won't give it up.  This diet is difficult enough without a caffeine headache.  In fact, I tend to up my caffeine in take on the first few days because of the detox headache.  Anything to help.  I usually resort to tylenol on these days as well.

Good luck!