With unstructured grids, lon and lat are not dimensions, but are 1-d variables specifying the lon,lat of each "column" of the unstructured grid.
Add to your existing ncl scripts, code such as:
lat1d = f[0]->lat
lon1d = f[0]->lon
res@sfXArray = lon1d
res@sfYArray = lat1d
This matlab routine shows how to plot results on an unstructured grid.
Another option for looking at the unstructured grid is to interpolate to a uniform grid (something finer than the finest resolution of your variable grid is recommended), and then use your standard plotting routines.