Drag the Audacity window to screen width. Near the top of the window a little to the right of center there will be a little pictogram of a microphone. Set the entry to the right of the pictogram to "Microphone - USB Audio CODEC."
Make sure that the entry to the right of the speaker pictogram is set appropriately for your setup (mine is Speakers - Realtek High Definition Audio) -- this will allow you to listen to your record as you record it.
That ought to do it. It will only see your turntable in the trace on the screen once you hit the "Record" button (the red dot). Until the needle drops, this will be a flat line.