Ok I have finally downloaded and installed Arctic.apk (thanks @stevov for the xda link). Now I have root privileges on my Kobo Arc. When I open SuperSU (v 1.42), though, I am asked to update the app, but both NORMAL and TWRP/CWM procedures will fail. Have any of you made it? Do you feel like sharing how? I am no android expert, so please do not be too geeky

Thanks for your help.