Associate a program

[arcoombes]

Hi, I am fairly new to Linux, and am running XnView MP Linux Version 0.90 64bits (Mar 5 2018) on Ubuntu 16.04 LTS.
I would like to be able to open an image file in Darktable directly from XnView. If I right click, go to "Open With .." and then "configure programs .." I can't add anything due to permissions?
Can anyone tell me how to configure this please?
Andrew.