I've tried it on my local computer and it has created a new product and responded with a JSON info about the new product (including its product_id):
http://devs.x-cart.com/en/basics/rest_api_documentation.html#creating-product
Do you see any error messages in the var/log/ directory?
Also, do you see any error messages in the JavaScript console in your browser when you open the URL?
Are you sure that you use the correct key (that is allowed to write data to the database)?