If you look at the end of the chrome cap, you'll see a plastic disc. Pry that disc off and there will be a black rubber plug that needs to be removed. That will reveal a zerk fitting.
READ THE AXLE LUBE INSTRUCTIONS CAREFULLY !!!!!! You have to jack the wheel and rotate it while using a HAND grease gun to SLOWLY inject lube into the zerk fitting. Otherwise, you stand a great chance of destroying the rear grease seal and having to pull the wheel/hub assembly to replace it.
Thinking out loud: If you're going so far as to jack the wheel off the ground, inject grease and possibly have to pull the hub anyway, why not just pull the hub and check the brakes and magnets while you're cleaning and lubing the bearings. It means new seals, but that's a very inexpensive cost compared to a new hub/spindle from bearing failure.
