I'll try to describe it.
There is a small machined area in the center portion of the Cardan joint. Some have described it as rubber. The photos are not really too clear and it is tough to see. In the middle of that area is what looks like a small metal disc, slightly shinier than the rest and probably 1/8" or so in diameter. If you press on the disc with a tool like an awl or the lube needle, you will see that it is actually spring loaded. That is the fitting. By pressing the needle against the disc, the grease is pumped past it into the cavity behind.
Older Cardan joints lack the fitting, being called "lifetime lubed". One on my '88 Ford Bronco lasted about 90K miles. When it went out it started chirping as it rotated, telling me it needed replacement.
I hope this is of some help.
Dan