Download the plugin archive and expand it (you've likely already done this).
Upload the Vote-the-Post directory to your wp-content/plugins directory.
Go to the Plugins page in your WordPress Administration area and click 'Activate' for Vote-the-Post. This will create the database table used by Vote-the-Post.
Congratulations! You've just installed Vote-the-Post and users can now rate your posts.
Optional: go into Options > Vote-the-Post to modify the number of times a particular IP address can vote per post or whether the voting interface and results appear at the top or the bottom of the blog post.