That zerk is at the base of the three point hitch control lever. It is under the operator station. If you get under the tractor in front of the rear tire and look backwards, you should see it. If you can't find it, I'll try and grab some pics later on.
Also, as far as greasing frequency, I just hit all the zerks whenever I fill up the fuel tank. That seems to be an easier interval to remember than every "x hours".