I've not been able to find a solution to this anywhere.
I'm using CS6 (creative cloud) on a win7 PC.
I've followed through all of the steps that are posted in a couple of the solutions above - editing MMDocumentTypes.xml and both of the Extensions.txt files. I've noticed that all of the suggested solutions refer to earlier versions of DW, not CS6.
LESS and SCSS files both open in DW following this, but I don't get any nice code colouring or hinting. Any help would be appreciated.