Maemo Mapper can't show the name of the street you are on, because it doesn't actually have any street data (e.g. Navtech). Maemo Mapper only has bitmap data and the data that comes from your GPS receiver (i.e. latitude, longitude, speed, heading, etc.). So it can plot your location on a bitmap, but it can't "read" that bitmap to interpret which road you're on (or near).

As for displaying "the name of the street that is to be your next turn", the routes that you download (via the "Download Route" dialog) will usually include "waypoints" that, as you get closer to them, will cause a popup message to appear instructing you what to do next (e.g. "Turn left at Main Street. Go 2.6 miles.").