Error while adding a new plugin (no replies)

I want to add a new plugin. I follow the instruction provided here Add a new plugin
but when I write this command : php composer.phar install I get those errors :

Quote
Your requirements could not be resolved to an installable set of packages.

Problem 1
- Installation request for guzzle/guzzle v3.9.2 -> satisfiable by guzzle/guzzle[v3.9.2].
- guzzle/guzzle v3.9.2 requires ext-curl * -> the requested PHP extension curl is missing from your system.
Problem 2
- guzzle/guzzle v3.9.2 requires ext-curl * -> the requested PHP extension curl is missing from your system.
- aws/aws-sdk-php 2.7.1 requires guzzle/guzzle ~3.7 -> satisfiable by guzzle/guzzle[v3.9.2].
- Installation request for aws/aws-sdk-php 2.7.1 -> satisfiable by aws/aws-sdk-php[2.7.1].

Please Help