Read this first!
If...
You don't have Java and/or JavaScript enabled. Java and JavaScript are blocked by Internet Explorer by default. For IE users, when the yellow information bar pops up at the top of your browser (it reads something similar to "This site is trying to run ActiveX controls on your computer"), click the bar and select "Allow Blocked Content". For users of other browsers, go into your settings (usually under Tools) and make sure that Java and JavaScript are enabled.
Bugs that have already been submitted.