The package they came in should be able to tell you if they are microwavable safe. If you don't see that don't place them in the microwave to avoid melting.
some ziploc cups are indeed microwave safe. you want to make sure you dont cook what your cooking for very long tho, and make sure that the item especially if its liquid has proper ventilation.
Putting any kind of plastics, ziplock or other wise, in the microwave, is not recommendable. Plastic is made out of all kinds of noxious chemicals, and when heated it is prone to releasing these chemicals into your food.