With jack started, in Audacity: Edit -> Preferences -> Devices: Select Jack Audio Connection Kit. Did you notice that JACK is an recursive acronym?do you know how to connect Audacity to JACK? It doesn't show up in JACK's connection menus.
Well, this is not all. Audacity is not very jack friendly. Its ports (portaudio) appear/disappear when you press/unpress the play button. Use the pause button as a workaround. For example, to record: Press Record, then press pause, make jack audio connections and finally unpress pause.
I am glad I helped.
Cheers! Pablo