I don't know, if there is a way to download maps to your PC and put them to the N900. Usually you have to download maps using the Manage Maps Dialog directly in Mappero using your N900. If you do not have a data flat on your mobile phone contract take care that you are using wireless lan for this. You should start with downloading a small area (maybe your hometown) in the zoom-levels you would like to have on your device. That will give you a feeling for the time it will take to download bigger areas.

T.