While I do not personally approve of being pressured to sell or promote anything that a person may have ethical issues with doing, I would advise you to be proactive before you refuse to sell the product.
1. Find out if it is required as part of your employment. If it is, you need to decide if you can live with this or need to move on. If you do decide that you can’t compromise on this, I would advise you to find other employment before making a stand.
2. While I do not like meal programs, I do see the value of having the option when it allows clients to get a reasonable meal in over something totally worthless. If you have to sell the product, I would sell it to clients as a transition system to better eating habits. You would focus on having clients work on doing a better job of selecting whole food sources at the grocery store while having the meal plan products as back up if they found themselves short on time to shop and prepare a better meal.
3. If the product does not contain ingridients that you definitely object to, you might consider trying it out yourself for a while. Integrate it into your own diet and see how you react/feel. I have done this a few times with diet programs as practical research. Just as my clients never perform an exercise that I have not tested personally, I never make recommendations for or against any thing that I haven’t tried or researched.
There will often be issues that test our personal ethics and values. It is reality. But we also have obligations and commitments that we must address as well. The balance can be a very difficult test for anyone.