When you choose to mark a waypoint using the “Mark” button (available in the lower left corner on all tools), it will default the lat/lon to your current GPS location. However if you know the specific Lat/Lon you want to mark, you can enter it there. The waypoint will then be saved at that location.
Alternatively, if you don’t know the lat/lon you can press and hold on the location on the map, and you will be given the option to mark a point at that location.
To Goto the point, you can choose “Go To” from the Compass Tool, or from the Map, and choose that point.