To add or delete a page, you need to be on the Campaign Builder

If you're working from the Page Builder, click the [ < ] Back Button to go back to the Campaign Builder

If you're working from the Dashboard, simply click the campaign you want to work on to open the Campaign Builder

From the Campaign Builder, click the [+] button found on the bottom right of the screen to add a new page.

From the Campaign Builder, click the "Page Actions" then choose "Delete" from the drop down