I believe the first line is showing the waypoint number that is currently chosen for navigation.
When you are adding a new waypoint, you just have to change the waypoint number on the 2nd line and enter coordinates on the WP/TGT page.
Some missions might have pre-assigned waypoints already on the 60L system and might be better to choose a starting number something like 50+
The easiest sequence would be:
1. Select WP/TGT page
2. Enter desired waypoint number to add by pressing the numer on the keypad. like 5 and 1 for 51 (this will change the waypoint number on the first line)
3. Press KYBD then press CLR to match the waypoint number on the 2nd line to the one you entered previously
4. Enter coordinates