In either VSL or on the board itself, press the Select button of the FX bus you would like to route. In the Fat Channel on the board, press the Assign button for your desired Subgroup. The button will illuminate, alerting you that the assignment has been made. You can also assign the FX bus using the Channel Tab in VSL.
For more information on Subgroup mixing and assigning FX buses to Sub output, please consult the Effect Group section in the Subgroup Mixing Tutorial in your user manual.