The barrel connector for the power output on the FOCUS monitors is a 3.5mmDia Barrel Jack and accepts a 1.1mmDia Center-Pin.
Pin 1 is Ground. Pin 2 is +DC. *Pin 1 is closest to the red dot on the LEMO connector.
You need to be careful when attaching a third party power source to your 502 monitor. It has an input range of 6.2V to 8.4V if you exceed 10v you can damage your monitor. Here is a good dtap to lp-e6 option that you can use with your 502 monitor: http://store.smallhd.com/store/500-accessories/DCA5-KIT