Make sure you have the plugin downloaded and saved as a zip file.
Procedure
In the Administration block, click on: Site administration > Plugins > Install plugins.
You will see the Plugin Installer page.
In the Install plugin from ZIP file section, click on:
Choose a file....
Figure: Choose a file...
The file picker window will open:
Figure: File picker window
Click on the button next to Attachment to select the
zip.file of the plugin.
Click on Upload this file.
Results
The system will upload the plugin automatically. After you
saved all of your changes, the theme plugin can be found under Site administration > Appearance > Themes > Theme selector.