I just did this and finally got it to work.

gain root access to your N8x0. type cp /usr/share/navit/navit.xml home/user/.navit

that will copy your xml file to your home/user/.navit folder make sure you type that exactly! Next I used vi to edit the xml file. just go to you home/user/.navit folder and type: vi navit.xml

the area you want to change is around the 7% mark. change the mapset to yes. I have my map located on my MMC card in the folder called map (Nokia N810). the location should look like this
<mapset enabled="yes">
<map type="binfile" enabled="yes" data="/media/mmc1/map/map name.bin"/>

If you are unfamiliar with vi I used the following website to help me out. message back if you are still in need of help.

http://www.cs.rit.edu/~cslab/vi.html