Call to undefined function curl_init() in .../api/HttpClient.php on line 75

What should I find and what should I edit in HttpClient.php on line 75 ?

private function _init($url,$token)
	{
		$this->_url = $url;
		$this->_token = $token;
		//init connection
{75}		$this->_curlConn = curl_init($this->_url);
		curl_setopt($this->_curlConn, CURLOPT_HEADER, false);
		curl_setopt($this->_curlConn, CURLOPT_NOBODY, false);
		curl_setopt($this->_curlConn, CURLOPT_RETURNTRANSFER, true);
		curl_setopt($this->_curlConn, CURLOPT_FOLLOWLOCATION, true);
		curl_setopt($this->_curlConn, CURLOPT_USERPWD, $this->_token);
		curl_setopt($this->_curlConn, CURLOPT_TIMEOUT, 15);
		curl_setopt($this->_curlConn, CURLOPT_SSL_VERIFYPEER, FALSE); // Validate SSL certificate
		curl_setopt($this->_curlConn, CURLOPT_SSL_VERIFYHOST, FALSE);
		curl_setopt($this->_curlConn, CURLOPT_USERAGENT, "FreshBooks API AJAX tester 1.0");
		return $this;
	}

This add-on is not working for me :((