You will notice two tabs at the bottom of the editor's page - HTML and Preview, ,. Click on the Preview tab and you will see a blank page. While the tabs are always visible, when in the Editor (HTML), you can click on the Preview button, , and when in the Viewer, you can click on the HTML, or Editor button, . You need to be in the Editor to use the following.
You need this for what follows.
The following appears on the page, with the cursor between the quotes, ready for you to type your message.:
(If you wanted more than a brief message then you would have used the Alert Wizard for multi-lines.)
Click on the Preview Tab or press F9 and you will see your code.
Click on the Preview tab again.
This time you get an alert box appear saying Hello.
You can preview your code by pressing the Preview button or F9.