Click the maps button on the toolbar. This initially shows just a list of the maps that cover the current center of the screen. If you have just started, there is probably just one map listed.
Click Digital Map Store. This now displays all the maps for the currently viewed location that are available for download. Some of these are free maps, but some must be activated before use. Your online account allows you to access the maps on multiple devices and PCs (iOS, Mac, Windows, and Android).
By default, the listing is just for topographical maps and small scale base-maps. If you are looking for nautical charts, air charts, or any other specialist maps, tap the Map Types button and turn on the type of mapping you want to access.
If you select a map that has to be activated, the Memory-Map for Mac application will prompt you to sign into your account. If you have used Memory-Map before you should sign in with the same name and password as you used previously. All the resources in your account are then available for activation on the Mac. If you are new to Memory-Map you will need to create an account.
When you have signed in, you will be prompted to activate the map.
If you have already purchased the map, it will show the number of activations available in your account and allows you to activate.
If you have not yet purchased the map you can do so by clicking the 'buy' button. When you complete the online purchase the resources are automatically added to your account and are immediately available for activation.
Advice for specific map types can be found here