Extract the zip file and just drop the contents in the wp-content/plugins/ directory of your WordPress installation (or install it directly from your dashboard) and then activate the Plugin from Plugins page.
After that go to options page 'Shorten2Ping' and fill the required information for your accounts, and customize the message template if you want.
Now you're ready, the plugin will notify Ping.fm or Twitter (at your choice) everytime you publish a new post (and Ping.fm will do the same to every site that you configured there, if you use this service).