Learn how to preview your campaign before publishing it
The preview button allows you to watch your campaign's position on the homepage.
(You are the only one who will be able to view it this way).
Make sure you set the campaign position on the left sidebar.
If you set a "Point position to element" or "In-page" position you will not be able to preview it.
On 'Test the campaign before publishing' article you can check how can you test the campaign and watch its correct position on your site.