I'm not sure if you managed to enter the co-ordinates into the Waypoint Manager screen or the Where To >> co-ordinates screen
This is how you would enter into the Waypoint managers screen, NOTE that these screens are from a GPSMap64 but your screens are probably similar
select Waypoint Manager then select a sample waypoint, now use the up, down, left and right keys to move around the screen, you will see an area highlighted (blue in the pictures), move the highlighted area onto the location area as shown in the left hand image, then select the 'Enter' key, the middle image will then be displayed, now move around this screen to select a digit to change and select done when completed
You can also move around the screen to change the waypoint Name and Icon etc. on GPSs with a touch screen you may be able to touch an area to select instead of using the arrow keys
when you want to go to this location select MAP (in the left hand image) to see the Waypoint on a map and GO to go to this location
The way you get to the location will depend on selections in Profile for 'Direct', 'Walking', 'Driving' etc. when navigation, you may also have a 'Change Route Activity' where you can also select 'Direct', 'Walking', 'Driving' etc.