You need to route it within the software.
From the FAQ page of the product support:
From the Preference menu, select Input Routing and then select the [DDJ-T1 Mic/AUX L（R）] input from the Input Aux menu. Set the Layout drop down menu to '4 Deck Full' you can access the Aux volume control from the right hand side of the crossfader bar. Customers are warned that there are no EQ, effects with this configuration. For full fader control, EQ and effects, please use the Live Input function on the Decks, such as the default Deck D.