In the Owners Manuel pg 37 it states “A note to StudioLive 64S users: The Matrix Mix option is only available on FlexMixes 17-32. Aux and Subgroup options are available for every FlexMix.”
In order to access the Matrix Mix capability go to a Flex Mix 17-32 (Access on the board by pressing the Flexmix Button two times.) You will then be able to assign it as a Matrix Mix.