Simplenote.el runs under Emacs 23 but it will not run properly under Emacs 22. If you cannot upgrade to Emacs 23 here are some workarounds that might help simplenote.el work under Emacs 22.
You have to install the package json.el, available at http://edward.oconnor.cx/elisp/json.el [from rbelew]
The :safe keywords in defcustom are not supported in Emacs 22. The corresponding lines must be deleted or commented out [from rbelew].
The function add-file-local-variable does not exist in Emacs 22. You can either copy-paste the corresponding code from files-x.el in Emacs 23, or replace (add-file-local-variable 'simplenote-key key) in the function simplenote-create-note-from-buffer by the following very naive code