Uploading maps on the website which use terrains from add-ons will not work! Starting with Widelands version 1.0 it is recommended to upload new maps within the game as an add-on. To do so start the "Add-Ons Packager" via "Main menu → Add-Ons → Tab Development → Launch the add-ons packager".
The map files have to be placed in the Widelands map directory to be found by the game. Check the Technical FAQ to find the map directory.