Warning--the monitor in your DKJr may be a Sega/Nanao, Kagi or similar that is a 100V monitor. Just like a Sanyo or Sharp 100V, putting it in your Pac-Man may cause damage to the monitor by running it at 120V.
Edit: Yup, you have a Kagi:
http://www.therealbobroberts.net/kagi.html I have 2 of these, one in a Sega Turbo and one in a Sega Cocktail. Both were running at ~100 volts. Please double check your voltage running into it before you connect it in another cab. The power inputs should be on a grey larger connector much like typical yoke wire connectors and be towards the top center of the picture closer to the very large black filter capacitor.
RGB = Red, Green, Blue signals.
VD = Vertical Deflection (aka Vertical Sync) - Typically combined with horizontal on a white white or may be Yellow or some other color if separate.
E = Earth (aka Video Ground) - Typically black when in a bundle with RGB.
HD = Horizontal Deflection (aka Horizontal Sync) - Typically White when in a bundle with RGB
Usually you can tie vertical and horizontal sync wires together and feed it into a composite sync pin or a horizontal sync pin and the monitor will separate them automaticaly.
But doublecheck what chassis that is before you connect it to the Pac-Man. The Pacman just has a 1:1 isolation transformer that outputs the same voltage as your wall (+/- a few volts). A regular Nintendo will run a 1:1.1 transformer stepping down 120V down to 100 volts or so.
Best bet is to disconnect the power cable from the monitor in the DK Jr machine and then use a digital voltage meter to check the voltage BEFORE connecting it into your pacman. Also meter the same on the pacman. If they're close (within 5%) then you're safe. If not post the numbers and/or more information about the monitor in your DKJr before you connect it up.
Your Pacman has a Wells-Gardner K4900 in it. From the right of the picture going left, it's Red/Green/Blue/Ground on the wide connector, then sync runs to the last 2 pins on the second connector.
So, in summary:
DK JR:
White - Composite Sync
Black - Ground
Red - Red gun
Green - Green gun
Blue - Blue gun
Pac-Man
Red - Red gun
Brown - Green gun
Yellow - Blue gun
Black - Ground
Black \ composite sync spread over both HSync and VSync pins
Black /
Sync in the synchronization signal that tells the monitor how fast to draw each line (horizontal) and how fast to draw each frame (vertical). On TV's it's combined with another color (component) or with all (s-video/composite). For RGB it is run over a separate line.