If you are creating a gradebook item or new assignment, you should be able to add the rubric there, Make sure your rubric is "Published" in the rubrics tool. If it is not, you will not be able to add it. If you do not see it aftter it is published, check with your LMS administrator. Your role permissions may need to be aligned.
Turns out that I needed to turn on Competencies in the Tools list and the option to add Rubrics showed back up.