The variable you're looking for is ispell-program-name.
Stuff this somewhere in your .emacs:
(setq ispell-program-name "/path/to/ispell")
Or use M-x set-variable, etc.
References:
From the sources to ispell.el
(defcustom ispell-program-name
(or (locate-file "aspell" exec-path exec-suffixes 'file-executable-p)
(locate-file "ispell" exec-path exec-suffixes 'file-executable-p)
(locate-file "hunspell" exec-path exec-suffixes 'file-executable-p)
"ispell")
"Program invoked by \\[ispell-word] and \\[ispell-region] commands."
:type 'string
:group 'ispell)
See also: http://emacswiki.org/emacs/InteractiveSpell
Whenever you can't find an Emacs function, remember C-h f (or a variable: C-h v). Entering ispell at the Describe function prompt tells you that ispell is an interactive compiled Lisp function in 'ispell.el'., and from there you can usually find what you're looking for.