if you "have already the programm" (this means the maps on your phone and the deb package installed [via xterminal is how i did it]) then you shouldnt have any problems. what is the error msg you get when you try and open the sygic application from the "more" menu in the n900?

if you already installed the .deb file al you should have to do is reboot your phone and then you should see the sygic mobile maps application in your phone in the "more" section of the phone