As abolishtheun pointed out, it's normally in /usr/sbin (at least it is on my Slackware boxes).
Just a comment on locating commands... Since typing the command in a shell resulted in "command not found" then "which" won't find it either. The which command searches the user's PATH for the command. Try using slocate (e.g. slocate useradd) or find (e.g. find / -type f -name useradd).