http://wiki.phathack.com/Cable_FAQ shows the part number for the cable you need (KHD-5L1).
Also, I think some Kenwood Sirius tuners have two connectors (pass through) so the tuner can be used in conjunction with the Phatbox. But some of the tuners don't (one connector). If your tuner only has one connector, you will have to abandon the Sirius tuner and connect the Phatbox directly to your head unit with the KHD-5L1, or buy a separate Kenwood switching box.
You will need the KHD-5L1 cable in any case. This is a DIN to Phatbox connector cable. The cable that came with your tuner should be a DIN-DIN connector.
Firmware and head unit can be selected in Phatnoise Music Manager or Media Manager.