There is a 2nd page to the Destination Input screen...press the arrow to change to the screen. Then there is a lat/lon button to input the coordinates.
Also be aware that the LR3 Nav takes degrees, minutes (and seconds?)
While some online maps (such as Google) just display degrees with a decimal (i.e. 32.4563N). The decimal portion will need to be converted to minutes by multiplying by 60.