( looks like i found another tutorial to do at the weekend.. will it never end? )
there are 2 types of wall blockers.
walls that are solid, and walls that can be shot through.. ( wall and wall s.t. )
they are wall tiles 620 and 621 respectively.
they look blank in the selector, and don't show up on the map..
to view them, i recomend first turning the hex grid on ( clik the hex button.. ).
then go to the tools menu, and select "toggle block obj view" to make the "hidden" tiles visible.
to place them, just "fill in" the hexes under the walls that are not red ( or don't already have a green w in them )
there are a few more invisible tiles that also show up in this mode, such as scenery blockers, scroll blockers, invisible blockers ( that stop the player, but don't show up on the pip boy minimap like the walls and scenery blockers do.. ), exit grid markers ( because the exit grid tiles also don't show on the minimap ), light scources and so on..
most of these are documented in the readme that comes with the editor, allbeit not very well..