This is a problem that sometimes plagues websites like ours that use Silverlight. If you run into this, it's often a browser security issue. You might want to try using a different browser first just to see if this could be it.
For example, in Internet Explorer 9, there's a feature on your Tools menu (press Alt+T to bring up this menu) called ActiveX Filtering. If this option is checked, even though you've installed Silverlight, it'll be disabled and websites like ours will think it's not installed and will therefore ask you to install it.
Another IE9 Silverlight blocker is the Add-ons menu, that you can reach via the Tools menu (Tools | Manage add-ons). Select "Toolbars and Extensions" in the left pane, and make sure you've selected, Show: All add-ons in the dropdown on the left. Then look for "Microsoft Silverlight" in this list. Make sure it's "Enabled", because otherwise you'll have the same problem.
One great way to see if Silverlight is properly installed is to visit another website that uses Silverlight 5, for example, Telerik's demo page.