I recently downloaded the latest WordPress onto my Mac.
Just to clarify, WordPress is not a desktop application. It will not run on your mac unless you have properly installed and configured a server environment to host it. - MySQL, PHP, Apache
I already have a wordpress account.
If you mean on wordpress.com, the two accounts (self-hosted .org software and free hosting .com blogs ) would pretty much be mutually exclusive.
I want to be able to modify my wordpress and have the changes appear on my actual wordpress that I created.
That probably won't happen in the synchronous manner I think you may be hoping for. Perhaps you may want to look into a desktop blogging client that allows you to create content directly from your mac and then publish your posts directly from the desktop.