Did you do the txt document correctly?
Naming it bigmap.txt
Putting the files in this order (example if it's a 3x2 overworld)
NPC Code:
"overworld_a1.nw","overworld_b1.nw","overworld_c1. nw",
"overworld_a2.nw","overworld_b2.nw","overworld_c2. nw"
Make sure the bigmap.txt is in the same folder as ALL your overworld levels.