The documentation needs updating - the Area coordinates are initialized based on a square surrounding the current center location (wide enough to fill the screen).

If you need something more precise, I suggest using something like Google Earth (on a PC) to locate the lat/lon of the corners of the rectangle that you want to download. The in-Maemo-Mapper alternative is to right-click on your desired corners and use "Show Lat/Lon" to get the latitude and longitude, which you have to memorize and enter into the text fields. Blech.