Please ensure you enable your subwoofer to ON as the default setting for this is OFF. This can be found in the Zones tab under the subwoofer module. Once this is enabled you can set the crossover setting, level, and profile (if powering via the following Even Zone).
Please refer to Page 24, Section 3.6 of the user manual for a detailed guide: https://anthemav.com/downloads/MDX_Manual.pdf