Once you have enabled the menu builder you can now build custom menus for your users.
Navigate to the menu builder and click the new menu button in the top right of the page. This will load up a new menu with all menu links available.
You are now free to edit the menu as you see fit, you can add new items, reorder items and change permissions.
Once you are happy with your menu you will then need to choose who it applies to and activate it. This can be done from the menu settings.
In order for the menu to be loaded, you need to tel uiPress who to load it for. This is done on a role or username basis.
Once you are happy with your menu and settings, save the menu and if you applied the menu to yourself then it will load for you when you visit a new page or refresh.