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->lat
lon1d = f->lon
res@sfXArray = lon1d
res@sfYArray = lat1d
Plotting unstructured grids with MATLAB
This matlab routine shows how to plot results on an unstructured grid.
Regrid output to regular 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.