This isn't an Emacs problem. You need to have the ispell program installed on your machine. ispell is available on homebrewis available on homebrew, so you can do:
$ brew install ispell That's probably all you need. If Emacs can't find ispell after you've installed it:
$ which ispell will tell you the path to the ispell executable.
Alternatively, aspell and hunspell are compatible and can be used instead.