Your Magento install is incorrect. According to this report it looks like you're missing the PHP MCrypt extension:
http://www.a2iwheels.com/report/?id=500049770619&s=default
Here's a few tips when debugging Magento problems:
First:
Enable debug/developer mode by uncommenting/adding the following lines to the Magento index.php file:
Mage::setIsDeveloperMode(true);
ini_set('display_errors', 1);
Second:
Setup the Magento exception log:
- In the Magento Admin, go to System->Configuration
- In the left column, click on Developer (under Advanced)
- If it’s not expanded, Click on Log Settings
- Select Yes from the “Enabled” drop down
- Click on Save Config
Note that you'll need to manually create the log files before Magento begins writing to them.
Third
Install FireBug and use it to catch AJAX errors - the single page checkout is essentially one complicated AJAX page.