Click on Teaching, then on Assessments. Select your assessment, then go to the General Settings tab and click Randomized Assessment. Enter the number of randomly chosen questions that you want to provide. For example, if you have 20 questions in total, but want your assessment to choose 5 of them randomly, you would enter 5 in the field. Click Update Assessment at the bottom when finished.
If you want to simply randomize the order of questions while including every question, set the Number of Randomly Selected Questions Per Assessment to the same number of questions that your assessment has.