There are several alternatives for retrieving maps, all of which require access to the internet. In the "Maps" menu, there is a "Manage Maps..." menu item that will bring up a dialog that allows you to download maps either by rectangular area or along your current route.

The other option is to just enable "Auto-Download", and Maemo Mapper will download the maps on an as-needed basis, as you zoom and pan around the map. Like the other options, though, this requires access to the internet, which means you must be connected to the internet while you browse.