In the PDF viewer in the right part of the screen, you can see a preview of the exercise that you are creating. To refresh this preview, save any exercise and the PDF will refresh automatically. The green bar in de bottom of the PDF viewer will show if the PDF is loading or not. If it is loading, a blue bar will move from the left to the right of the bar.
Sometimes something is wrong with the question you created. In this case, the PDF will not compile and give an error. Most of the times this has something to do with equations or with LaTeX. Try to look at your last changes to find out what the problem might be.