Hi Angela: You are correct. Apparently it's not possible to create multiple answers in a short-answer question. In that case, you probably need to use the "multiple-answer" multiple-choice question type:
http://docs.moodle.org/22/en/Multiple_Choice_question_type#Multiple-answer_questions
I've never been a huge fan of multiple-choice questions b/c I think it's too easy for students to guess the right answer (and very hard to create credible distractors). Maybe you could submit a request for Tim Hunt or one of his folks to create a new question type? (multiple-answer short-answer question). Somewhere in the Moodle docs there is probably info on how to do this (I can't remember at the moment). Thanks. - Peter