Can't Grease

I know this sounds bad, but I can't find the grease nipple for my trailer bearings. Its a Zieman trailer, the sticker at the front of the trailer says it has Bearing Buddies, but it lies. Upon research, these are definitely not Bearing Buddies. Anyone have the same type? That silver ring in the middle around the plastic center is spring loaded and pushes in. I tried to pry the center cap out but didn't have any luck and didn't want to wreck anything. I obviously haven't greased them since I bought the boat in '09 and don't want push my luck any further. As far as I know they are still fine though, and I'll keep a close eye on them in the mean time. All the Zieman manual says is to refer to the information provided in the Bearing Buddy axle booklet, which I obviously don'y have. Thanks for any help.