WordPress and IE9- Compatibility Issues

Hello,
My website is not viewing correctly in IE9, you are not able to see the nav bar and other items. Is there something else I can do, other than telling all of our customers to click the broken page icon to left of the refresh button at the top of the browser?

I know I need to update my version of WordPress, so that might solve the issue, but I also have another site running on this same WP version and that site displays correctly. Could it be the theme?

My site: http://www.gorillanuts.com/

Thanks in advance.