There never was a '_UI.dll' included in the quick install package.
I've verified that the v1.2.3 UI can be used according to the install doc included.
There was one error in that version of the wrapper doc. that had the wrong web.config section name -- it s/b
My expectation was that the testing UI package would be 'wrapped' ( or portions extracted from it) into whatever application you would create so a UI.dll wouldn't be of benefit.
If installed per instructions, you can reference the testing UI by doing...
The current v.1.2.4 beta2 will be the 'live' version before the weekend is done, so you might want to grab that one when available.
Hope that helps,