Microwave oven grill heating element not glowing
I remember having one of these in a kitchen I worked in. I too was confused by it. 

When using the oven as a microwave the element stays off. When grilling, the element used to cycle on and off as the internal temp rises and falls. It also went off if the door was opened. It used to glow a dark orangey-red, only for a moment. It was difficult to see with the internal cabinet light on. However, it generated heat most of the time without an obvious visible glow. You can't just turn the grill on. You need to set a time as you would in microwave mode. There was no easy way to pre-heat the grill function. 

In my opinion the unit was a waste of time and I only used it as a microwave. Sorry probably not what you wanted to hear.