I'm using 'Expert' ROM (CM10.1) 4.2.2
I couldn't get it to keep the screen on while charging as I use it in desktop mode while at work and charging.
Seems the 'Developer' options in 'Settings' was missing!
Here's how to get it back.
Settings > About phone>
and quickly touch several times (about 10 times) on "Build number".
You'll see a toast saying "You're a developer now".
Then you'll find the stay awake option under "Developer options".
Excellent ROM by the way.......