It is generally not necessary to install either ispell.el or flyspell.el, because they both come packaged with most installations of Emacs. They are elisp files that do not need to built -- Emacs is designed to read elisp files such as ispell.el and/or flyspell.el.
Instead, what needs to be installed is a third-party executable such as aspell or hunspell. The language dictionaries will also need to be installed after the main executable is installed.
Essentially, ispell.el and flyspell.el (with help from Emacs) are front-ends for aspell or hunspell.
NOTE: OSX ships with an older version of Emacs. Instead of using an older version (e.g., /usr/bin/emacs), consider using a current version built --with-ns: http://emacsformacosx.com/ For more information on different builds that are available for OSX, the following link has some informative write-ups: https://emacs.stackexchange.com/questions/271/what-is-the-difference-between-aquamacs-and-other-mac-versions-of-emacs