If you want to install a extensions you have three official ways to do this:

1. Install via extensionmanager

This is the simplest way to install a extension. Just open the extension window and search for a extension - press the 'Details' button to open the detail window. Now you can simply click "install" to install this extension.

Note: For this way you need to have allow-url-fopen activated for php (more information).

2. Install via extension code

A extension code is the id of a extension. Each extension has it's own extensions code and is therefore unique.

Just browse over the extensions page and go to the extensions website, copy the extensions code and paste it into the top right field of the extensions window in your administration.

Note: For this way you need to have allow-url-fopen activated for php (more information).

3. Install via installation file

Instead of entering the extension code to the field you can click the choose button and choose a installation file. You can download the installation file on the extension website (Example) via the "download" button. Just upload this file to you server and choose it in the extension window.

This is your solution if the php option allow-url-fopen is not activated on your hoster and therefore step one and two doesnt work.