I believe you need to have the SID (System interface device) to plug it into your computer so that you can update the firmware. You can download the software from the digital aquatics website and their website will walk you through how to update firmware as well as provide info on setting up & programming your AC2.
The manual for the RK2 can be found here: RK2 Manual
According to the manual you will need the version of MyReef for the RK2 to update along with a serial cable that plugs into the temperature probe port. To stay connected to MyReef there is a splitter for the temperature port.
I got all of this from the manual. I haven't owned a RK2, I'm more familiar with the newer version of MyReef and the RKE, RKL.