In "Editor: set current editor
syntax", I see "C++" listed, and the default behavior file seems to associate it properly:
{:name "C++" :exts [:cpp :mm :cc] :mime "text/x-c++src" :tags [:editor.cpp]}
Does setting the mode manually manually work for you? If not, do you get any console errors; if so, does it continue not to be associated in a fresh copy of LightTable?