Hi,
I am little confused. Are you trying to enable the DEV mode, or to enable the CSS&JS Optimization? I think you are trying to enable the CSS&JS optimization...
CSS & JS compression that comes with the T3 framework has been disabled on SportMart, because it was causing overload in the server (does more harm than good)
I recommend you to use this plugin instead;
extensions.joomla.org/extension/jch-optimize
to compress&optimize CSS and JS files. It is a freeware plugin and works like a charm. That plugin is my favorite Joomla plugin.
Regards.
Support