Adding an odor treatment is recommended. If you download the owners manual you will see where the tank is loaded in theory. What is almost a certainty is that the tank will be close to wherever the pump out gunnel/deck fitting is located. For owners manuals search for Grady White owners manuals and go from there.
Most likely yours is a sea water flush but it could be fresh water. If sea water, there is a seacock that needs to be opened. If you system has overboard discharge option, there will be extra plumbing and a seacock for that operation. Note also, that if you have a combo ,pump out or overboard discharge) there are laws and regulations about where you can pump out and how you must prevent unauthorized pumpouts. Often that is accomplished bi using some method to prevent an unauthorized person from operating the overboard discharge feature either by locking or tie wrapping the discharge lever or in some instances a key switch to prevent operation.