The error message is trying to tell you that Emacs started the external program aspell
in a subprocess, which is good, but that it couldn't find its dictionary file, which is bad.
Try typing M-x ispell-change-dictionary RET SPC
to see if there are any dictionary files that Emacs knows about, and select one of them.
If that doesn't work, then there is something wrong with your installation of flyspell. What to try next depends on the version of Emacs you are using (the terminal version that comes with OS X, a newer terminal version installed via Homebrew or MacPorts, the Cocoa version, or Aquamacs). On my machine, for example, I am running Cocoa Emacs 24 under Snow Leopard, and flyspell.el
comes preinstalled, but to actually get it to work I had to install the aspell
package using Homebrew (which provided the aspell
executable and its dictionary files). Can you give us more information about your environment (OS X version, Emacs version, etc.)?