What version of Silverlight do I need to install?

My company has an inteal webapp to track some of our clients' information. This webapp uses Silverlight, but whenever I try to log in to it, I get the following error:

[IsolatedStorage_Operation]
Arguments:
Debugging resource strings are unavailable. Often the key and arguments provide sufficient information to diagnose the problem.
See http://go.microsoft.com/fwlink/?linkid=106663&Version=5.1.41212.00&File=mscorlib.dll&Key=IsolatedStorage_Operation

The link in the message provides the explanation for the error and offers this solution:

To see these descriptive error messages on your local machine and obtain other debugging resources, please install the latest version of Silverlight for Developers on your Windows or Mac computer.

I have tried installing Silverlight for Developers (version 3) as provided in the link (for Windows). I had to manually uninstall (via appwiz.cpl) my current version in order to do so.

After installing SfD, trying to open the app fails because I don't have the most recent version of Silverlight:

But clicking Install Now takes me back to the first error!

I have also tried to find SfD on Microsoft's Download Center, to no avail. Perhaps I am looking in the wrong place?

How can I get the right (latest) version of Silverlight for Developers?