Check your theme options. I viewed the source code and found this comment:
<!-- CHECK FOR REMOVE_SIDEBAR CUSTOM FIELD -->
So this leads me to believe the theme author has provided for single column content.
The other thing you can do is go into the core.css file and add this somewhere at the top:
God do I hate other people's code. That code is a mess; there's no reason to have a gazillion stylesheets going on there. It makes simple things like this so hard to change. Anyway, HTH.