Super Simple Question From an Absolute Beginner

I am a first time DM and purchased the "Dungeon of the Mad Mage" bundle after seeing how intuitive the toolset appeared to be on the Dungeons & Dragons official playthrough on their YouTube. My question is super simple, how do I access the content? I've created a new game, and enabled the add-ons, but when I launch the game I only get a blank grid rather than any of the maps included in the bundle.

The module should have everything you need.  If you also bought the map addon pack (intended for people that have the physical module and just want the maps/decks), and installed it on top of the module, then you wiped out all the npcs and set up that the module provided.  The addon pack has that warning on the page: Important Notice: The Waterdeep: Dungeon of the Mad Mage - Map & Card Deck Addon is not designed to be added into the core Dungeon of the Mad Mage adventure module, as these elements already exist within the module. Attempting to import them into the core module will result in your current maps being overwritten by the addon's maps.

Modules must be selected at the time of game creation. Did you select DotMM when you created the game?

OK, very cool. I deleted and re-created the game making sure to select the module and not to add any add-ons. Both of your replies were enormously helpful and a solution was as embarrassingly simple as I expected it might be. 

The difference between modules and addons, and where your purchases appear often confuse folks. There has been a greater emphasis in recent times to spell things out better on the purchase page, but there's a lot of room for improvement.