The bearings you have may be the Dexter E-Z Lube bearings--check with Dexter Axle, or just google for E-Z Lube. Tons of info on the internet.
But, yes, you use the zerk to add grease and it works well--you should probably have the wheel off the ground so you can spin it while adding grease. It is not like a Bearing Buddy which is designed to keep water out of boat trailer bearings. Many have suggested NOT to use an auto/cordless gun, but only a manual one to keep from getting too much grease in unintentionally.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired