The throttle block is the plastic that the throttle lever is attached to. It's on the handlebars.
If you are leaking oil, you will need to start at the oil tank and work your way to the carbs and find where it's leaking. It's a simple system, a line goes from the tank to the pump and two lines go from the pump to the fittings under the carbs. A leak should be pretty evident. You may have to take the carbs off to do a complete job of looking...