found this topic here, with a non-google search 
you´e not alone m8! you did everything right, but missed to enable the developer mode in the end.
go to extension-tab, then enable “developer mode” (right upper corner). Then just drag&drop the downloaded “video-downloadhelper.crx” there and voilá it´s installed.
My friend, the gamepplay is like this:
if there´s a really handy feature, then sooner or later it´ll get hidden or just cut off, because the endgoal is to create as much chaos as possible. its just another way, to keep us busy, for not (beeing able to) reach out and think about the real problems… Even now, I cannot wrap my head arround the fact, most browsers disabled the “BACK”-key for “return page”…
sorry for the rant…it seems my unability is more like a disability…oh sheeeit…, now the tinfoil is burning…g2g…
anyway, hope you now can use this wonderfull extension in Brave WITHOUT the need off logging in.
best regards
hanZdampF