Is this a newer (blue) machine or an older (green) one? In either case, the treatment is the same, but the method is slightly different. If it is an older machine, remove the honing wheel and use a 19mm or 3/4" wrench to remove the drive wheel. Slide the main shaft with the grindstone attached out of the frame and lubricate the two black nylon bushings in the frame. A Teflon based spray lubricant works well, as does any type of waterproof silicone grease.
If you have a newer machine with the EzyLock, it will be easier to take the stone off and pull the shaft out the other direction. Otherwise, the procedure is the same, lubricate the two nylon bushings and re-assemble.