Where do I get SilverlightUIAutomationHelper.dll?

We have a Silverlight solution that worked fine on our old computers. It was set up for coded UI tests, so the projects reference Microsoft.VisualStudio.TestTools.UITest.Extension.SilverlightUIAutomationHelper.

Then we got new machines, and evidently there's some obscure bit of configuration that we didn't get set up properly on the new machines, because now the reference shows a warning icon, and the Error List window shows the following warning:

The referenced component 'Microsoft.VisualStudio.TestTools.UITest.Extension.SilverlightUIAutomationHelper' could not be found.

I found this post that gives the path where this DLL is expected to be found, but that directory doesn't exist. On my 32-bit dev VM, I do have a C:\Program Files\Common Files\microsoft shared\VSTT\10.0 directory, but it doesn't have a UITestExtensionPackages subdirectory.

What am I missing? How do I install this DLL?