[Plugin: W3 Total Cache] Cloudfront option not available on nginx configuration?

Hi, I’ve just moved my site over to a server running nginx, and have slowly been ironing out the various config issues. However, when it comes to re-enabling cloudfront as the CDN, i’m not getting the option to do so from the general settings tab, either as pull or push… they’re both greyed out: http://screencast.com/t/K7tgZbKQM

Any idea how i can get these options re-enabled?

thanks in advance.

http://wordpress.org/extend/plugins/w3-total-cache/