PC Robots.txt automatically creates a virtual robots.txt file for your blog. Your robots.txt file can be easily edited from the plugin settings page.
The easiest way to install the plugin is from the Plugins, Add New page in your blog admin area.
If you click the click the Search menu link and type in something like "robots.txt" hopefully you will see PC Robots.txt in the list. Click the "Details" link and you should see a link or button to install the latest version of the plugin.
Alternatively, you could download the zip file for this plugin to your computer, and then go to Plugins, Add New, Upload, and browse to the plugin zip file you downloaded then click the Install Now button.
Once you have the plugin installed and activated, you'll see a new PC Robots.txt menu link under the Settings menu. Click that menu link to see the plugin settings page. From there you can edit the contents of your robots.txt file.