Last time I checked activities are not added to the gradebook if they don't have a grading method. If this has not changed in 2.1.2 (and afaict no reason for it to change) you should be able to find the forum in the gradebook once you assign a grading method and then you should also be able to place it in the desired grade category.
But since you don't want to assign a grading method ("I don't want the instructor or student to rank anything") you could simply add a gradebook grade item to stand for the forum activity and serve as the place holder for the activity grade.
hth