You will be redirected to a window with all the available question types, such as Multiple choice, Fill the gap, Ordering, Drag and drop, Free text and Randomized (2). After you have created a series of questions (without being "added" to the test) (3), click to add a Randomized one (4).
Type the Question name (5) and click on Add (6) to include the questions to the Randomized question and click on Save (7).
You are now back to the Test page and you click to add the randomized question to the test (8).
You will notice a little arrow next to the Added tag . Clicking on it you can select the number of the questions that will be randomly selected from the Randomized question you just added (9).
Now, every time a user clicks to take the test the system will automatically randomly select the number of questions you have selected and generate a random test.