How to install a translation for a WP plugin?

This Plugin uses load_plugin_textdomain(), which by default uses the Plugin’s root directory as the path for the translation files.

So: simply add the translation files you downloaded to the Plugin root directory, and you should be good to go.

Edit

And you’re using WordPress in es_ES?

No, I just want to translate that plugin. It generates a form with fields such as “Your Name” “Your Email” “Your Remark” and I need that form translated in Spanish.

If you actually want WordPress to use those translation files, you have to tell it to use them, by changing WPLANG from en_US to es_ES in wp-config.php.