When I try to adduser or useradd it says command is "not found".
Some sources suggest adding /usr/sbin to PATH, but it's already there.
Some sources suggest first logging on as root, but su returns "user root does not exist".
For background, I'm trying to do stuff with my Asus router, which is running Linux version 2.6.36.4brcmarm (root@asus) (gcc version 4.5.3 (Buildroot 2012.02)