Glow plugs run on DC current, so they are not polarity sensitive.
The stem of the plug is connected to the internal coil, which is made of platinum. The bottom end of the platinum coil is connected to the plug body. You can see the connection when you look at the bottom end off the plug.
Connect one side of the 1.5Volt DC power source to the stem of the plug, and the other side to the hex shaped plug body, and the element will glow.
Your link does not work. Most of us use Rossi R-8 or MC-9 plugs.
The engine case should be drilled and tapped 1/4"-32 UNEF for the glow plug threads. This will provide the ground for the glow plug circuitry.
Here is a link for the glow plug wire that will attach to the center terminal of the plug.

www.electrodynam.com/rc

ugc.jpg Universal Spring-loaded Glowplug Connector
$6.95
KGC0100
Universal Glowplug Connector. Spring-loaded contact. Pull on wire to open clamp and snaps on most glowplugs and glow engines. Great for a Scale look! Beware of look-alikes! The KGC0100 uses special high-temperature solder for demanding installations. Works even in turbine engines!

I tried to upload this image of a Rossi R-8 plug after "teasing" the element out of the plug body, but it didn't work...hope it comes out now....