Hi. looks like there is no SBUS out on that RX ?, sbus is not mandatory but…there is many options of Spektrum compatible receivers. Good luck with your build.
From Cube manual, “Spektrum DSM/DSM2/DSM-X Satellite Receiver
For a Spektrum DSM, DSM2, or DSM-X Satellite receiver, connect to the SPKT port on The Cube.”
You can read about all related info from ardupilot site.
http://ardupilot.org/copter/docs/common-connecting-the-radio-receiver-apm2.html