To make sure you've got the USB interface set to 32 channels:
Setup menu, Card tab:
set to 32 in/32 out.
To connect the USB input to the 32 channels:
Routing menu->Home tab:
this is a list of possible inputs that can be assigned to the channels. This is done in blocks of 8:
In the input 1-8 column, select the row 'Card 1-8'. This assigns channel 1-8 of ...