I'm not sure the way I do this is "approved," but I use the ASIO4ALL kernel wrapper, and set the ASIO4ALL input pointer to the USB turntable. Then I use JRiver's ASIO Line in, pointing at ASIO4ALL. It works seamlessly.
I wouldn't recommend using ASIO4ALL for any other purpose (like for your actual playback), but it works just fine to simulate an ASIO input for line in purposes.
Another alternative that I've used in the past is to use a free recording suite like audacity, with the "playthrough" option checked. Then use JRiver's "loopback" feature to re-route the sound. This method is laborious and kind of fiddly, though, which is why I switched to the first method above.
Edit: I should note, I'm doing this in MC 18, so you may not have the "loopback" option.