Hey.
Seems you have some questions like I had in the beginning. but this topic has (almost) all the answers.
You mayneed to by a 12v barrel jack cable with only a male plug to branch out the 12v coming in and going into the LCD Driver board. something like this:
http://www.ebay.com/itm/10pcs-Surveillance-CCTV-Camera-DC-12V-Power-Cable-Male-Connector-/110759673207
The jumper cables going from the ATXRaspi to the Raspberry (to send shutdown/boot signals) as per LowPowerLabs recommendation should be good quality to avoid problems, and if powering Pi via GPIO from ATXRaspi, you should also use high grade jumper wires.
depending on the power source you select, your female dv jack in should match the power source. most say the plug is either 5.5mmx2.1mm or 5.1mmx2.1mm so you should get a female jack panel mount that matches. something like this:
You then onlu need to drill 2 small holes on the back panel of the Picade, one to fit the switch, another to fit the dc socket.
As for artwork, try and have a look at this topic:
http://forums.pimoroni.com/t/new-artwork-for-picade-cabinet/2618/278
Hope this helps (and that I got it right) and dont forget to post pics of your completed setup :)