export PATH=$PATH:/dir/where/ispell/binary/located/ for [BA]SH
-- or --
setenv PATH $PATH:/dir/where/ispell/binary/located/ for [T]CSH
But... How did you install it? If it was RPM, it should put the binary somewhere already in the $PATH... What is your distro? If RPM-based, get the full list of ispell package (rpm -qpl ispell) and check whether its' binary (usually /usr/bin/ispell) is listed there...