The Master Cleanse is really not a good thing to do if (prior to the cleanse) you aren't already having at least one - unassisted - bowel movement per day. It's really best to get that issue resolved prior to doing the Master Cleanse.
You could possibly resolve this issue with several different approches. I would make sure to up my daily water intake to at least 1/2 my weight in ounces of water (if you weigh 180lbs, you should be drinking at least 90oz of water - in addition to all other liquids). Also a series of good probiotic suppliments should help. Adding Kefir to your daily diet is also good. Check out the bowel cleansing forum for a lot more information.
Because the Master Cleanse requires your body to be regularly eliminating toxins, if you have issues with daily elimination, it is advised to take care of that first - otherwise you could get really sick and uncomfortable. Don't be discouraged... I previously had issues with this too. Curezone has fantastic forums that can instruct you and really help you get past this. Once you are having at least one daily bm (without the use of laxatives or enemas), you will be a great candidate for the Master Cleanse.