Are you using the equipment to enhance your abs that are already there or to try and get rid of a belly?
If it's the former then these machines work, the latter I'm afraid require hard work, exercise and diet - the machine will just work the abs but if they are hidden under a layer of fat you will not see any results.
The majority view is that you cannot spot train certain parts of the body i.e you cannot get rid of belly fat just by exercising the abs, fat is lost from the whole body when you burn more calories than you consume.