Originally posted by Tinkster
You have to "su -" to root, the file is in /usr/sbin
You have to read more carefully. The quote solves your problem completely. Trust Tinkster
You have two choices:
1) the program you want to start is included in your path
2) you start the program giving the full path
ad 1) Starting su with the option " -" gives you the full path root would have when logging in to the system after startup. So directories like /usr/sbin, /sbin or /opt/kde/bin become part of roots path variable after doing su and will be searched for commands.
ad 2) the other possibility is to start /usr/sbin/usermod (giving the full path) or doing ./usermod in the /usr/sbin directory (giving relative path)