How to Install WordPress.org Plugins Manually
You can install plugins to your WordPress.com blog manually. As an example, install the Subscribe to Comments plugin.
In the search box at the top of the page, enter the keywords Subscribe to Comments and then click the Search Plugins button.
A search results page appears.
Locate the Subscribe to Comments plugin on the search results page and click the plugin name.
The Subscribe to Comments page opens in the WordPress Plugin Directory, where you find a description of the plugin, as well as other information about the plugin.
Click the Download button to download the files.
Follow the download instructions in your brower's dialog box.
Locate the file on your hard drive and open it with your favorite decompression program.
If you're unsure how to use a decompression program, refer to the documentation available with the program. A subscribe-to-comments folder, along with other documentation, appears on your hard drive.
Connect to your Web server via FTP and open the /wp-content/plugins/ folder.
This folder contains all the plugins that you currently have installed in your WordPress blog.
Upload the entire subscribe-to-comments folder into the /wp-content/plugins/ folder on your Web server.
Depending on the number and size of each plugin you use, this step may take a minute or two.
Log in to your WordPress.com blog.
The main WordPress.com page appears.
Click the Installed link on the Plugins menu.
The Manage Plugins page opens, listing all the plugins installed in your WordPress blog.
Locate the Subscribe to Comments plugin.
Because you haven't activated this plugin, it’s located in the Inactive Plugins section at the bottom of the Manage Plugins page.
Click the Activate link to the right of the plugin name.
The Manage Plugins page refreshes, and the Subscribe to Comments plugin is now listed in the Currently Active Plugins section at the top of the page.