You can browse questions by section or search for questions based on question text, title, keywords, or the question ID. Any question you have used before or created is stored in your personal bank.
After clicking "search" or "go", a list of questions will appear at the bottom of the page. Clicking on a question name will allow you to preview that question. When you find one you would like to add, click on the blue arrow in the far right column. Once you have selected all of the questions you want to add to the assignment, click "Done adding" on the right.