By default, NavigX for iPad is set to convert between Latitude and Longitude and Ordnance Survey Grid References (UK). This is changeable by using the Settings App on your iPad.

To change the conversion type, go into the Settings app and scroll down to NavigX for iPad. Touch that cell and then select the 'Conversion Type' option. Change that to your desired reference system. When you go back into NavigX for iPad, you'll be able to convert into your newly selected units.

NOTE: You may need to restart NavigX for iPad in order to invoke the change.