You may or may not need to clean the cell, depending on your water chemistry. The self cleaning feature can take care of it if your CH level isn't too high. But if CH has crept up at any point you may still need to clean it manually. You should do a visual inspection of the cell plates and see if they look clean, or if they are coated with some white chalky stuff. If there is white, they need to be cleaned.
The cell is the last place on the unit the water goes through, where the water comes out. There should be unions on both ends and a cable connecting the cell back to the box that has the control panel on it.