I've used these types of validation on optional modification that, if opted for, need additional information. If the additional information is a value from an entry field, you need to go to that attribute and default it to "0". Once that's done, you need to create a validation with the conditions of "if this optional modification is opted for, and this addition info equals a 0, then fire." You will then white an action message saying something along the lines of "if this optional modification is opted for then you must provide the additional info".