In which folder do I place the w3s files [for custom maps] in order to properly play a custom map?

Thanks in advance.

Place it in the Darklords folder, run the Scenario editor program (its in the Darklords folder too) and pick an "unpack scenario" option. Select your w3s file from the list. then close the editor and run Darklords. The new map will appear in the Misc list.