Please install vqmod : vqmod
You need to paste all codes stated in the install.txt file on the root folder index.php , .htaccess file and set a secret key from admin panel and enabled the extension.
If you want to use short URLs like: http://yourdomain.com/api/rest/products or http://yourdomain.com/api/rest/products</a>, you need to enable RewriteEngine, rename your .htaccess.txt to .htaccess, and paste all RewriteRules from install.txt.
Opencart API edited answer November 25, 2015