QuickPost is a plugin consisting of a bookmarklet which will be added to your links toolbar, and a blog editor triggered by the bookmarklet.
Download the zip file.
Copy the "quickpost" directory to your /wp-content/plugins directory.
Activate the plugin.
Select the default categories for your QuickPost posts from the QuickPost tab which is added to your "Plugins" administration page.
To start using the plugin, you will have to add the QuickPost bookmarklet to the links toolbar of your browser. To do that, please follow the instructions on the QuickPost Settings page.