With the current software you will have to use the “Waypoints, Routes, Tracks” manu (under the Nav menu tab). Go to the “Options”, then select “New”, than select “New Waypoint”. From there you will need to set the name and Lat./Lon. As well as some other information.
If you have more than one or two you may want to instead use the HumminbirdPC program and manually type them into your computer, then upload the data to an SD memory card and from there into your unit.