To assign a menu a publication, navigate to the publication settings for your publication and select the Menu tab.
Publication settings are the tabbed section on the Manage screen of your publication.
At the top you can select a menu from the Active Menu field. All menus that have been created in your account will be available in the Active Menu drop-down list.
If you have publications that are part of a suite of related documents, apply the menu to all of the related publications in their publication settings.