If you did not install MWS to default location, this gets a bit more complicated. First locate where MWS was installed to. Once you locate that, make sure everything is in the correct sub-folder. Assuming you installed to:
c:\location
All the custom sets MUST be in the folder c:\location\custom sets
The master base file, "warcraft.mwbase" MUST be in the folder c:\location
The theme file, "default.mwtheme" MUST be in the folder c:\location\themes
Since I do not know where you installed MWS, change "c:\location" to whatever directory you installed MWS.
Must get ready for Conts... (sounds much worse than Nats... lol) - Northern General