If there's one installed it should be near the water pump. If you're not sure where that is turn it on and follow your ears, you'll likely need to lift a seat or remove a panel to access it. Once there you'll hopefully see a "T" junction on the water line near the pump with about a 3' piece of hose hanging off of it. Lots of good write-ups here about best practices for winterizing.
Also, make sure you've bypassed your water heater and possibly your water filter if you have one before attempting to fill the system with anti freeze. Otherwise you'll end up wasting more than you need to.
2007 Shamrock F19 hybrid - 2010 (22 nights), 2011 (24 nights), 2012 (11 nights)
2013 Surveyor Select 235RKS - 2012 (6 nights), 2013 (32 nights), 2014 (26 nights), 2015 (37 nights), 2016 (14 nights)