Hello. I am also encountering the same issue where i can't merge vertices together. I tried your Polygon > Combine tip but nothing happens. I get an error msg saying "polyUnite needs at least 2 polygonal objects."

Just to double check :

1) Convert NURBS to polygons
2) Delete away the nurbs and position the polygons back in place
3) Right-click and select vertex
4) select 2 or more vertices and select Edit Polygons > Merge Vertices.

Did I miss out anything ? And where should the Polygon>Combine come in ?

By the way, this site is awesome ! I'm from Singapore and I simply love this tutorial site ! I will recommend this site to my friends. Keep up the good work !