The question library allows you to add to a new questionnaire questions that you have previously created.
All this operations are available in Step 2: Questions.
To save a question, simply click on the little floppy drive that can be found on the question when it is selected.
To call a saved question, click on "Saved question" in the panel on your left hand-side. You then have access to a little search engine. Enter the corresponding key-word (words contained in your question) and select the question from the list that appears then.