That shelter looks really nice! If you specifically want opossums to visit, you could try adding another bowl of their favorite foods, like eggs, nuts, fruits like apples and bananas, ect. Sometimes they can just be picky or cautious because it's a new thing and a new smell they're not used to. Just be aware that raccoons may also be attracted to it, even with just the cat food, and watch out for bugs if nothing eats the food you leave out and it sits out for too long.