I have the house with all the rooms because I did the POWER OF ATOM quest. I went and bought the Cookamatic 3000 from the girl at the store. I don't know what to do with it!!!!

Just keep it in your inventory, and all food and water will automatically have less rads. Weighs 7 pounds though.... kinda heavy for my taste.

You're thinking of a food sanitizer. I'd guess the OP means the Cook-o-matic 3000 added by the http://planetfallout.gamespy.com/mods/312/Vault-Dwellers-Survival-Cooking-Guide mod, in which case the OP just needs to click on it and assuming they have a recipe and the ingredients they can make a meal.