Similar to the last post, this is also not a feature of the theme. You’d need to make your own CSS customizations for this to be possible. This is honestly not really going to be an easy task because like many dynamic designs, we do not know how many buttons are going to be in the main menu, and so they’re simply floated left which allows the user to have more freedom in adding them.
For CSS changes, it’s a good idea to use a tool like firebug or Google Chrome’s built-in developer tools to locate them and then put the edits either in your child theme or in the “Custom CSS” option on your theme options page.
Also, here’s a helpful tutorial on how to use Firebug – http://vimeo.com/20917974. The only thing I’d suggest not doing is actually making edits to the theme’s CSS files, as it shows at the end of that video. However, the video serves as a good introduction to Firebug and how it can be used to find what CSS is effecting certain areas of your site without ever digging around in any files.