mike schubert welcome aboard
Check in the engine compartment for a Y-valve in the waste line prior to the holding tank. If it has one it may be in the wrong position. If not someone has by-passed the tank and you will need to reconfigure the waste hose.
To empty the holding tank you use the waste clean out located on the rear of the boat, probably starboard (right) side. Most marinas will have a pump-out station. It is a big vacumn system that suck the stuff out of the tank.
1997 2400 SCR
5.7 Vortec / Bravo 2