You must install or ask your web hosting to install CURL

If you are not on your local machine, their is high probability that your web host won't allow CURL, so you I recommend you if you want to use chevereto to switch to another webhost.

If you are on wampserver follow theses steps :

1) Left click on the small icon in the task bar.
2) click on php
3) click on php extensions
4) click on php_curl (the second one in wamp server 2 portable)