This article is related to PromoteMyPlace Version 2 only. Not sure which version of the application you're using? See Which version of PromoteMyPlace am I using?
From the ‘Website’ dashboard, choose ‘Menus - Main Menu’
Select the ‘+’ under the main menu item that the submenu will be beneath
Choose Type ‘Page’ and select the page from the dropdown list
Add a label for the name of the menu item
Select ‘Add Item’
The sub-menu will be below the main menu
Save your changes
Once you’re ready to put your changes on the live site you will need to publish the website changes by clicking the orange 'Publish Changes' button at the top of the screen.