Map data is automatically downloaded as needed by Ovi Maps. As you search for locations or move about, it downloads the data for the immediate surroundings. Think Maemo Mapper, but vector, so no separate zoom levels to worry about.

Should you wish to reduce data usage on the go, or have maps available beyond the reach of wireless data, you can optionally download ahead of time from a selection of large, regional map data packs. Think Wayfinder. One of the "Suite" products is supposed to assist with this task for the N900, but as I understand it, it is flaky. As with Wayfinder, manually downloading and unzipping the large files is easy though.