The Google Maps website (maps.google.com) no longer supports loading KML files via a URL parameter like that ("?q=http://example.com/myfile.kml"). An alternative is to make a simple web page that loads a map via the Google Maps Javascript API, and add your KML file as a layer. Documentation and example scripts are available at these links:
https://developers.google.com/maps/documentation/javascript/kmllayer
https://developers.google.com/maps/documentation/javascript/kml
Also, as the other answer indicated, the DropBox link you're using opens the DropBox web page for the file, and does not go directly to the KML file. You need a URL that will simply download the file. I think you can do that easily, since it looks like when you change the "dl" parameter to be "dl=1" that it will directly download the KML file. I haven't tried hosting KMLs on DropBox, but I think that could work with the Maps API.